libgtk2: remove useless post-patch hook

There used to be a post-patch hook to apply a patch only on the host
variant of libgtk2. However, after the reorganization of the patch
naming policy, this was no longer working properly, so
dd1848bc changed this by a patch that
works on both the target and host variants.

We can therefore get rid of the useless post-patch hook.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>