disas/nanomips: Remove helper methods from class

Helper methods from NMD class like NMD::renumber_registers,
NMD::decode_gpr_gpr4... etc. are removed from the class. They're now
declared global static functions.

Following helper methods have been deleted because they're not used by
the nanomips disassembler:
- NMD::encode_msbd_from_pos_and_size,
- NMD::encode_s_from_s_hi,
- NMD::neg_copy

Global functions used by those methods:
- nanomips_dis
- sign_extend
- extract_bits
have also been defined as static global functions.

Signed-off-by: Milica Lazarevic <milica.lazarevic@syrmia.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220912122635.74032-6-milica.lazarevic@syrmia.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>