[X86][NFC] Remove class ADCOXOp(RR|RM) and redundant let statements

1. Remove these two classes b/c opcode is changed from 0xF6 to 0x66
   after promotion, then the classes become useless.
2. Remove `OpSize = OpSizeFixed` b/c the default value is OpSizeFixed.
3. Remove `let isCommutable = 1` b/c ADCX/ADOX is not VEX-encoding,
   we can not apply VEX3ToVEX2 optimization for it and the compiler
   never emits it.
4. Remove predicate `HasADX` due to no pattern

This patch is to extract the NFC in #76319 into a separate commit.