scalar-crypto: Remove DECLARE_RV*_ONLY macros from encoding.h

- These are no longer needed since none of the scalar crypto instructions
  overlap their encodings any more.

- There is more code to be removed in relation to letting Spike handle
  overlapped RV32/64 instructions, but this will be done in subsequent commits
  and PRs so that the downstream scalar-crypto work can be un-gated.

 On branch scalar-crypto
 Changes to be committed:
	modified:   riscv/encoding.h