Revert "Fix #2971: add reminder events to the bug history"

This reverts commit f4753ec5be52ddae8518c3c1a26caea0c88e61c8 following
discussion on the mailing list [1].

[1] http://thread.gmane.org/gmane.comp.bug-tracking.mantis.devel/4330

Conflicts:
	bug_reminder.php
Create reminder bugnote even if no message text provided

When adding a reminder, a bugnote was actually NOT created when the
reminder text is empty, even if $g_store_reminders = ON.  This was due
to bugnote_add() returning false when $p_bugnote_text is blank as the
code did not take REMINDER type notes into consideration.

Fixes #15744