x86: simplify table-referencing macros

First of all it is entirely unclear why THREE_BYTE_TABLE_PREFIX() was
introduced by bf890a93a7c4. Nothing uses the .prefix_requirement values
from the two relevant entries.

And then having VEX_Cn_TABLE() and friends take arguments is misleading.
These aren't used (or pointlessly used in the case of VEX_C5_TABLE); the
respective table index is decoded from the insn (or implied in the case
of VEX_C5_TABLE).