Remove unused libpython3 dependency
gtk-doc only requires the interpreter, and Meson itself already depends on python3 >= 3.5, so that check we useless and breaks build on CI docker that doesn't contain libpython3-dev.
gtk-doc only requires the interpreter, and Meson itself already depends on python3 >= 3.5, so that check we useless and breaks build on CI docker that doesn't contain libpython3-dev.