Delete superfluous implicits.Collections class

In the course of upgrading the Frontend codebase to Scala 2.13 we came
across the `implicits.Collections` class, and had trouble updating it
for Scala 2.13: #25190 (comment)

It turns out that we can _delete_ this class with the Scala 2.13
upgrade, rather than have to update it, which is nice!

To reduce the size of the Scala 2.13 upgrade PR, we first removed
every use of `implicits.Collections` that we could while still under
Scala 2.12: #25226

This commit gets rid of `implicits.Collections` altogether. At this
point, all `implicits.Collections` has in it is the code to add a
`distinctBy()` method onto standard Scala collections - this was
introduced November 2012 with #263,
but Scala 2.13 has a new built-in method that works the exact same way!
So we can delete the Frontend implementation.

https://www.scala-lang.org/api/2.13.x/scala/collection/Seq.html#distinctBy%5BB%5D(f:A=%3EB):C

admin/app/football/model/PA.scala

@@ -1,11 +1,10 @@
 package football.model
 
 import pa.{Season, Team}
-import implicits.Collections
 
 import java.time.ZoneId
 
-object PA extends Collections {
+object PA {
 
   val competitionNames = Map[String, String](
     ("100", "Premier League"),

commercial/app/controllers/BookOffersController.scala

@@ -17,7 +17,6 @@ class BookOffersController(
 ) extends BaseController
     with ImplicitControllerExecutionContext
     with GuLogging
-    with implicits.Collections
     with implicits.Requests {
 
   private def booksSample(isbns: Seq[String], segment: Segment): Seq[Book] =

common/app/common/TrailsToRss.scala

@@ -27,7 +27,7 @@ object RssDates {
   }
 }
 
-object TrailsToRss extends implicits.Collections {
+object TrailsToRss {
 
   // The CAPI blocks we would like consumers to request to build our item intro text from
   val BlocksToGenerateRssIntro = "body:oldest:10"

common/app/layout/Front.scala

@@ -14,7 +14,7 @@ case class Front(
     containers: Seq[FaciaContainer],
 )
 
-object Front extends implicits.Collections {
+object Front {
   type TrailUrl = String
 
   def itemsVisible(containerDefinition: ContainerDefinition): Int =

common/app/model/facia/PressedCollection.scala

@@ -5,7 +5,6 @@ import com.gu.facia.api.{models => fapi}
 import com.gu.facia.api.utils.ContainerBrandingFinder
 import com.gu.facia.client.models.{Branded, TargetedTerritory}
 import common.Edition
-import implicits.CollectionsOps._
 import model.pressed._
 import org.joda.time.DateTime
 import services.CollectionConfigWithId

common/app/services/repositories.scala

@@ -5,14 +5,13 @@ import com.gu.contentapi.client.model.ContentApiError
 import com.gu.contentapi.client.model.v1.{ItemResponse, SearchResponse, Section => ApiSection}
 import common._
 import contentapi.{ContentApiClient, QueryDefaults, SectionTagLookUp, SectionsLookUp}
-import implicits.Collections
 import model._
 import org.joda.time.DateTime
 import play.api.mvc.{RequestHeader, Result => PlayResult}
 
 import scala.concurrent.Future
 
-trait Index extends ConciergeRepository with Collections {
+trait Index extends ConciergeRepository {
 
   implicit val context: ApplicationContext
 

common/app/views/support/MostPopularTags.scala

@@ -5,7 +5,7 @@ import model.Tag
 import model.pressed.PressedContent
 import implicits.FaciaContentFrontendHelpers.FaciaContentFrontendHelper
 
-object MostPopularTags extends implicits.Collections {
+object MostPopularTags {
 
   /** A descending list of the tags that occur most frequently within the given items of content and how frequently
     * they occur

sport/app/football/feed/agent.scala

@@ -88,7 +88,7 @@ trait Fixtures extends GuLogging {
   }
 }
 
-trait Results extends GuLogging with implicits.Collections {
+trait Results extends GuLogging {
 
   def footballClient: FootballClient
   def teamNameBuilder: TeamNameBuilder