The validation of the timezone field should not occur, especially
when it is automatically cleaned. Timezones can be volatile, we
must try hard to fallback on real timezones and must not lose reset
the values arbitrarily.
"There is absolutely no need to change $CFG->timezone and user timezones
in database - the timezones may come and go. If you change the value in
upgrade or on the fly you would not be able to get it back. This is the
reason why I implemented the "invalid timezone" thing in server and
user settings instead." - Petr Skoda (MDL-49684)
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