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.

gio/.gitignore

@@ -1,20 +0,0 @@
-gapplication
-gconstructor_as_data.h
-gdbus
-objectmanager-gen-org.gtk.GDBus.Example.ObjectManager.Animal.xml
-objectmanager-gen-org.gtk.GDBus.Example.ObjectManager.Cat.xml
-objectmanager-gen.[ch]
-gdbus-daemon-generated.[ch]
-gio
-gio_probes.h
-gio-marshal.[ch]
-gio-public-headers.txt
-gio-querymodules
-gioenumtypes.[ch]
-glib-compile-resources
-glib-compile-schemas
-gnetworking.h
-gresource
-gschema-compile
-gsettings
-xdp-dbus.[ch]