Remove the rest of showcommands, dist goal

showcommands is all handled in soong_ui, make/kati/ninja never see the
argument.

Remove the dist goal, since we'll never pass it to ninja, only to Kati
as a modifier.

Remove DUMP_%, since that's not used anywhere (it appears to be in our
code search via the NDK build system)

Test: m -j showcommands dist
Change-Id: I0d1498128caece685e98c2c2b5b0c3f545da0e11