Shrink the debug library size even more

Pass remove_webcore_debug_symbols=1 to gyp to reduce
the library size.
In case the symbols are needed CONFIG+=webcore_debug
can be passed to qmake.

Change-Id: I383aa3c60bfe6a0dc5344d37334b25810087d4e9
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>