systemd: remove useless options for mips4

systemd: remove useless options for mips4

Looking back the history, we had problem with systemd on qemumips64
which is also related to compilation flags. We solved that by using
tweaking FULL_OPTIMIZATION for mips64 to have "-fno-tree-switch-conversion
-fno-tree-tail-merge".

Now systemd has been upgraded to 234, and we don't have the above problem
any more, thus removing these flags.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Armin Kuster <akuster@mvista.com>