build: Remove */.gitignore files

Since out-of-source-tree builds are now used after switching to meson,
we don't need .gitignore files in the source directories to ignore
build artifacts.

This fixes build errors when doing a meson build after an autotools
build, because generated files such as gio/xdp-dbus.c won't show up in
a `git status`, or be removed by a `git clean -f`, and so it won't be
obvious that such files need to be removed for the meson build to
succeed.

po/.gitignore

@@ -1,3 +0,0 @@
-POTFILES
-*.gmo
-glib20.pot