disas/nanomips: Remove NMD class

disas/nanomips: Remove NMD class

NMD class has been deleted. The following methods are now declared as
static functions:
- public NMD::Disassemble method
- private NMD::Disassemble method
- private NMD::extract_op_code_value helper method

Also, the implementation of the print_insn_nanomips function and
nanomips_dis function is moved to the end of the nanomips.cpp file,
right after the implementation of the Disassemble function.

Signed-off-by: Milica Lazarevic <milica.lazarevic@syrmia.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220912122635.74032-10-milica.lazarevic@syrmia.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>