[C2y] Remove support for _Imaginary (#97436)
WG14 N3274 removed _Imaginary from Annex G. Clang has never fully
supported Annex G or _Imaginary, so removal is pretty trivial for us.
Note, we are keeping _Imaginary as a keyword so that we get better
diagnostic behavior. This is still conforming because _I makes it a
reserved identifier, so it's not available for users to use as an
identifier anyway.
