disas: nanoMIPS: Remove functions that are not used

Some functions were not used at all. Compiler doesn't complain
since they are class memebers. Remove them - no future usage is
planned.

Reviewed-by: Aleksandar Rikalo <arikalo@wavecomp.com>
Reviewed-by: Stefan Markovic <smarkovic@wavecomp.com>
Signed-off-by: Aleksandar Markovic <amarkovic@wavecomp.com>