contrib: qtsvg: remove no longer needed patch

From testing, it seems that the qtsvg build retrieves zlib.h from $(PREFIX)/include/QtZlib/zlib.h. Its directory is specified with the -I flag. The $(PREFIX)/include directory is specified with the -isystem flag, which has lower priority than -I. So $(PREFIX)/include/zlib.h has the lowest priority, and the patch is not needed anymore.

qtbase also includes plain <zlib.h>, but qtbase already has -I flags pointing directly to the vendored zlib directory in the source tree.