macros2: Deprecate GNOME_MAINTAINER_MODE_DEFINES

The modules which it deprecates are all ancient and unused. Many of the
modules which use GNOME_MAINTAINER_MODE_DEFINES don’t make use of the
DISABLE_DEPRECATED variable it generates anyway.

If a module really wants maintainer mode, they should use
AM_MAINTAINER_MODE. Otherwise, they should just eliminate this call
entirely.

https://bugzilla.gnome.org/show_bug.cgi?id=735417