prep_cif.c: Remove unnecessary ifdef for X86_WIN32

ffi_prep_cif_core had a special case for X86_WIN32, checking for
FFI_THISCALL in addition to the FFI_FIRST_ABI-to-FFI_LAST_ABI range
before returning FFI_BAD_ABI.  However, on X86_WIN32, FFI_THISCALL
already falls in that range, making the special case unnecessary.
Remove it.