tools_def: Remove duplicated -Os

tools_def: Remove duplicated -Os

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=2928

commit 17bd834eb5fd ("BaseTools: Factorize GCC flags")
makes GCC48_ALL_CC_FLAGS inherit from GCC_ALL_CC_FLAGS.
GCC_ALL_CC_FLAGS contains the '-Os' flag.
The latest flag in a command line overrides the previous
optimization option. This allows more specific build
configuration to override the inherited '-Os' flag.

If a build configuration includes GCC48_ALL_CC_FLAGS,
hard-coded '-Os' options are not necessary anymore.
Remove them.

Reported-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>