package: remove references to symbols TARGET_{ps3,pxcab}.

Target pxcab and ps3 were removed from maintaince in r34764 and r34765
respectively.

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43850 3c298f89-4303-0410-b956-a3cf2f4a3e73