configure.ac: remove GLib deprecations workaround
configure.ac: remove GLib deprecations workaround

We are now free of usage of any features which are deprecated in the
minimum version of GLib that we depend on (2.50).

mvollmer/cockpit,mvollmer/cockpit,martinpitt/cockpit,cockpit-project/cockpit,martinpitt/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,martinpitt/cockpit,mvollmer/cockpit,mvollmer/cockpit,cockpit-project/cockpit,martinpitt/cockpit,mvollmer/cockpit,martinpitt/cockpit,cockpit-project/cockpit