Cleanup Mk/Uses/gnome.mk.

• Remove the use of comp_DETECT.
• Remove all comp_DETECT variables.
• Remove a use of comp_DETECT outside of gnome.mk
• Remove py3gobject3.
• Have ports depending directly on devel/py-gobject3 use gnome to do it.

PR: 224618
Submitted by: mat
Exp-run by: antoine
Sponsored by: Absolight
Differential Revision: https://reviews.freebsd.org/D13550