fix mingw-w64 build by removing the __WKHTMLTOX_UNDEF_QT_DLL__ hack

This was broken since cd6331b1aed95d3810fbb2a02517f8e2273d69ba due to
usage of QT resources. When building the library project, QT_DLL was
being defined which caused linkage issues. Earlier, this was worked
around by defining __WKHTMLTOX_UNDEF_QT_DLL__ which in turn caused
QT_DLL to be undefined in each source file, but this does not work
for the source file generated by rcc. A more sane approach is to
simply define QT_NODLL in case of static builds, which ensures that
QT_DLL is not defined at all via mkspecs/features/qt.prf