macros2: Remove GNOME_COMMON_INIT

It’s been deprecated for a while, and apparently no modules (which I
have checked out, at least) still use it.

https://bugzilla.gnome.org/show_bug.cgi?id=735416
doc-build: Remove doc-build infrastructure

This pertained to the old GNOME documentation infrastructure, used
before Mallard existed. This isn’t used by any modules we care about
any more. Most modules which set USE_COMMON_DOC_BUILD do so erroneously.

https://bugzilla.gnome.org/show_bug.cgi?id=729931
macros2: Remove support for gnome-doc-utils

Remove support for checking for the GNOME_DOC_INIT macro in configure.ac
and calling gnome-doc-prepare if found. gnome-doc-utils has not been
used in any modules we care about for a long time.

https://bugzilla.gnome.org/show_bug.cgi?id=729931