ui/Makefile.objs: delete unnecessary cocoa.o dependency

ui/Makefile.objs: delete unnecessary cocoa.o dependency

Delete an unnecessary dependency for cocoa.o; we already have
a general rule that tells Make that we can build a .o file
from a .m source using an ObjC compiler, so this specific
rule is unnecessary. Further, it is using the dubious construct
"$(SRC_PATH)/$(obj)" to get at the source directory, which will
break when $(obj) is redefined as part of the preparation for
per-object library support.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>