package/Makefile.in: remove BR2_TARGET_OPTIMIZATION from TARGET_CFLAGS

Since the toolchain is always wrapped and the wrapper already passes
BR2_TARGET_OPTIMIZATION (through BR_ADDITIONAL_CFLAGS), there is no longer
any need to pass it in TARGET_CFLAGS as well.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>