Remove Xbitmanip instructions

[aswaterman]

This change is motivated by the fact that Zicond extension support is broken, as pointed out in riscvarchive/riscv-zicond#12, because the ratified Zicond opcodes overlap the unratified Zbt opcode for CMOV.

We gave the unratified bitmanip instructions a two-year grace period in Spike, which I think was generous. And when some of these instructions are eventually revived, their implementation can be found in the git history.

[ptomsich]

I'll follow this up with a disasm.cc change to correctly recognize the czero.{eqz.nez}, as that had been missed on the original submission.

[aswaterman]

Thanks.