x11/libxcvt: drop USE_GITLAB to get non-FreeBSD mirrors

Not using USES=xorg-cat to avoid cruft:

• unused by xorg-server after ports 88b0ae2bb9c6
• USES=pkgconfig is unused by libxcvt
• USE_XORG=xorg-macros is unused with USES=meson
• USES=pathfix is unused with USES=meson
• EXTRACT_SUFX=tar.bz2 is unused by new releases/projects