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.