Remove duplicate weak definition

The weak pragma was assigned twice to the bl2_plat_handle_post_image_load
definition both in plat/common/ and in plat/arm/common/ this was an error as
it should have only have been defined in plat/common

Change-Id: Id85e14c699cb09ed998d1677f2a172e760911918
Signed-off-by: Daniel Boulby <daniel.boulby@arm.com>