Fix #19649 by using filterInScopeM in rnFamEqn
Previously, associated type family instances would incorrectly claim to
implicitly quantify over type variables bound by the instance head in the
HsOuterImplicit
s that rnFamEqn
returned. This is fixed by using
filterInScopeM
to filter out any type variables that the instance head
binds.
Fixes #19649 (closed).