[X86][NFC] Remove EVEX2VEXOverride/NotEVEX2VEXConvertible

Remove these two classes and put all the entries in X86 EVEX compression tables
that need special handling in .def file.

PR #77065 tries to add entries that need special handling for APX in
.def file. Compared to setting fields in td files, that method looks
cleaner. This patch is to unify the addition of manual entries.