Deprecate g_unix_mount_monitor_set_rate_limit()

Deprecate g_unix_mount_monitor_set_rate_limit() and turn it into a
no-op.

This function doesn't behave as advertised.  It only controls rate
limiting for filesystem-based monitors.  It has no impact over reporting
mount changes on Linux, for example, because those are based on polling
for changes in /proc (which doesn't use filesystem monitors).  It also
has no impact on Mac OS because a library interface is used there.

This was added in https://bugzilla.gnome.org/show_bug.cgi?id=521946 in
order to be used by HAL, which is effectively dead.  udisks no longer
uses this code at all.

https://bugzilla.gnome.org/show_bug.cgi?id=742599