gplugin/gplugin

Cleanup some minor Meson issues

2021-10-03, Elliott Sales de Andrade
3880a5d25cf2
Cleanup some minor Meson issues

We don't use gtk-doc any more, and the minimum is 0.56, so no need to keep
older checks.

Testing Done:
Compile.

Reviewed at https://reviews.imfreedom.org/r/987/
The following individuals have contributed to this project.
Ankit Vani <a@nevitus.org>
EionRobb <eion@robbmob.com>
Elliott Sales de Andrade <quantum.analyst@gmail.com>
Gary Kramlich <grim@reaperworld.com>
ivanhoe <ivanhoe@fiscari.de>
Jason Scherer <schereja@gmail.com>
John Bailey <rekkanoryo@rekkanoryo.org>
Mike Ruprecht <cmaiku@gmail.com>
Olaf Hering <olaf@aepfle.de>
Richard Laager <rlaager@pidgin.im>
Sorvival <sorvival@tutanota.com>