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.

sim/testsuite/configure

@@ -1823,7 +1823,6 @@ esac
 
 
 
-sim_common=yes
 sim_igen=no
 sim_arch=
 case "${target}" in
@@ -1914,10 +1913,6 @@ case "${target}" in
        sim_arch=v850
        sim_igen=yes
        ;;
-   *)
-       # No simulator subdir, so the subdir "common" isn't needed.
-       sim_common=no
-       ;;
 esac