RISC-V: Zvkh[a,b]: Remove individual instruction class

RISC-V: Zvkh[a,b]: Remove individual instruction class

Currently we have three instruction classes defined for Zvkh[a,b]:
- INSN_CLASS_ZVKNHA
- INSN_CLASS_ZVKNHB
- INSN_CLASS_ZVKNHA_OR_ZVKNHB

The encodings of all instructions in Zvknh[a,b] are identical.
Therefore, we don't need the individual instruction classes
and can remove them.

This patch also adds the missing support of the combined instruction
class in riscv_multi_subset_supports_ext().

Fixes: 62edb233ef5 ("RISC-V: Add support for the Zvknh[a,b] ISA extensions")
Reported-By: Nelson Chu <nelson@rivosinc.com>
Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>