Remove unnecessary extendTyVarEnvFVRn function

The `extendTyVarEnvFVRn` function does the exact same thing as
`bindLocalNamesFV`. I see no meaningful distinction between the two functions,
so let's just remove the former (which is only used in a handful of places) in
favor of the latter.

Historical note: `extendTyVarEnvFVRn` and `bindLocalNamesFV` used to be
distinct functions, but their implementations were synchronized in 2004 as a
part of commit 20e39e0e.