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

This reverts commit 3d34fba1c1ecfa703134d69f11a44de1ec9f4e7e following
discussion on the mailing list [1].

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