libgtk3: remove patch adding automake subdir-objects option

The patch libgtk3-0002-automake-add-subdir-objects-option.patch fixed
dozens of warnings from automake by adding subdir-objects option, but
with subdir-objects option automake seems to try making gtk/native/
even if gtk2-dependency is enable (see gtk/Makefile).

Fix error:

make[5]: Entering directory '/home/eksel/buildroot/output/build/libgtk3-3.12.2/gtk/native'
Makefile:506: ../.deps/native_update_icon_cache-updateiconcache.Po: No such file or directory
make[5]: *** No rule to make target '../.deps/native_update_icon_cache-updateiconcache.Po'.
Stop.

Signed-off-by: Hadrien Boutteville <hadrien.boutteville@gmail.com>