sim: mips: drop unused micromips igen logic

sim: mips: drop unused micromips igen logic

This code appears to be unused since it was first merged.  When
micromips was enabled, it was via the "MULTI" config, not the
"MICROMIPS" config, and the multi configs have sep vars.  Since
nothing sets SIM_MIPS_GEN=MICROMIPS in the config, all of this
should be unreachable, so punt it to simplify.  Further, the
SIM_MIPS_MICROMIPS16_FLAGS & SIM_MIPS_MICROMIPS_FLAGS settings
rely on sim_mips_micromips{,16}_{filter,machine} variables that
are never set in the configure script.