sim: drop --enable-sim-cflags option

sim: drop --enable-sim-cflags option

No other sub directory provides such a configuration option, so
drop it from the sim dir as well.  This cleans up a good bit of
code in the process.

If people want to use custom flags for just the sim, they can
still run configure+make by hand in the sim subdir and use the
normal CFLAGS settings.