Clean up .gitignore

Meson enforces a separate build dir, so we no longer have to
care about build artifacts in the source tree. Same applies for
all the generated crap autotools like to spread around.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/261>

.gitignore

@@ -1,29 +1,3 @@
-ABOUT-NLS
-Makefile
-Makefile.in
-Makefile.in.in
-aclocal.m4
-autom4te.cache/
-config/
-configure
-config.log
-config.status
-data/*.json
-m4/
-po/*.header
-po/*.sed
-po/*.sin
-po/Makevars.template
-po/POTFILES
-po/Rules-quot
 po/gnome-shell-extensions.pot
-po/stamp-it
-staging/
 zip-files/
-
 *~
-*.gmo
-metadata.json
-*.desktop
-*.gschema.valid
-*.session