Replaces other Applicative.pure(()) with .unit

[satorg]

A follow up for #4557 – other uses of Applicative.pure(()) are found (with grep) and replaced with .unit.

Note, that there are other places where .pure(()) is in use, however those calls don't belong to Applicative and to replace them the .unit methods have to be added accordingly:

./free/src/main/scala/cats/free/FreeInvariantMonoidal.scala:104:      def unit: FA[S, Unit] = FreeInvariantMonoidal.pure(())
./tests/shared/src/test/scala/cats/tests/IndexedReaderWriterStateTSuite.scala:111:      val rws: ReaderWriterState[String, String, Int, Unit] = ReaderWriterState.pure(())

[johnynek]

Thanks for improving the code.