Remove subs type optimization causing find method issues (#6532)

## Description

This PR fixes #6491

## Checklist

- [x] I have linked to any relevant issues.
- [ ] I have commented my code, particularly in hard-to-understand
areas.
- [ ] I have updated the documentation where relevant (API docs, the
reference, and the Sway book).
- [ ] If my change requires substantial documentation changes, I have
[requested support from the DevRel
team](https://github.com/FuelLabs/devrel-requests/issues/new/choose)
- [x] I have added tests that prove my fix is effective or that my
feature works.
- [ ] I have added (or requested a maintainer to add) the necessary
`Breaking*` or `New Feature` labels where relevant.
- [x] I have done my best to ensure that my PR adheres to [the Fuel Labs
Code Review
Standards](https://github.com/FuelLabs/rfcs/blob/master/text/code-standards/external-contributors.md).
- [x] I have requested a review from the relevant team or maintainers.

---------

Co-authored-by: Joshua Batty <joshpbatty@gmail.com>

sway-core/src/decl_engine/ref.rs

@@ -147,19 +147,13 @@ where
         ctx: &SubstTypesContext,
     ) -> Option<Self> {
         let decl_engine = ctx.engines.de();
-        if type_mapping.source_ids_contains_concrete_type(ctx.engines)
-            || !decl_engine.get(&self.id).is_concrete(ctx.engines)
-        {
-            let mut decl = (*decl_engine.get(&self.id)).clone();
-            if decl.subst(type_mapping, ctx).has_changes() {
-                Some(
-                    decl_engine
-                        .insert(decl, decl_engine.get_parsed_decl_id(&self.id).as_ref())
-                        .with_parent(decl_engine, self.id.into()),
-                )
-            } else {
-                None
-            }
+        let mut decl = (*decl_engine.get(&self.id)).clone();
+        if decl.subst(type_mapping, ctx).has_changes() {
+            Some(
+                decl_engine
+                    .insert(decl, decl_engine.get_parsed_decl_id(&self.id).as_ref())
+                    .with_parent(decl_engine, self.id.into()),
+            )
         } else {
             None
         }

test/src/e2e_vm_tests/test_programs/should_pass/language/generic_trait_constraints/src/main.sw

@@ -58,6 +58,21 @@ impl MyFrom<Struct4> for Struct3 {
     }
 }
 
+// call an associated function through generic constraints
+pub trait SizeInBytes {
+    fn size() -> u64;
+}
+
+impl SizeInBytes for u64 {
+    fn size() -> u64 {
+        8
+    }
+}
+
+fn call_size<T>() -> u64 where T: SizeInBytes {
+    T::size()
+}
+
 fn main() -> bool {
     let s1 = Struct {data: 1_u64 };
     assert_eq(s1.data.my_add(1,2),3);
@@ -72,5 +87,8 @@ fn main() -> bool {
     let s4: Struct3 = Struct4{data:42}.my_into();
     assert_eq(s4.data,42);
 
+    // call an associated function through generic constraints
+    assert_eq(call_size::<u64>(), 8);
+
     true
 }