bindings: drop build time message on missing ruby dependency

bindings: drop build time message on missing ruby dependency

The usual make message on everything being up to date is:

	make: Nothing to be done for 'all'.

However, since

commit d038b9320949c79503fe4b76870aaabf279405ab
Author: David Bremner <david@tethera.net>
Date:   Mon Jun 1 09:08:59 2015 +0200

    build: integrate building ruby bindings into notmuch build process

if one doesn't have the ruby dependencies installed, the message has
been:

	Missing dependency, skipping ruby bindings

Restore the usual behaviour by dropping the message. It's redundant
during build anyway, since the configure script already outputs:

	Checking for ruby development files... No (skipping ruby bindings)