Remove `Path::is_ident`.

It checks that a path has a single segment that matches the given
symbol, and that there are zero generic arguments. It has a single use.

We also have `impl PartialEq<Symbol> for Path` which does exactly the
same thing *except* it doesn't check for zero generic arguments, which
seems like an oversight. It has numerous uses.

This commit removes `Path::is_ident`, adds a test for zero generic
arguments to `PartialEq<Symbol> for Path`, and changes the single use of
`is_ident` to instead use `==`.