Fix FP for `boxed_local` lint in default trait fn impl
Fix FP on default trait function implementation on `boxed_local` lint.
Maybe I checked too much when looking if `self` is carrying `Self` in its bound type.
I can't find a good test case for this, so it could be too much conservative.
Let me know if you think only detecting `self` parameter is enough.
Fixes: #4804
changelog: none