(bug 29472) kill $wgUseDynamicDates

Remove $wgUseDynamicDates and everything related to it.

I left DateFormatter::reformat() alone, since it might possibly be
used elsewhere, and to be honest I'm afraid to touch it.

Change-Id: I609db8471c14e5e5946916f085d2ee5b96204d81
Merge "(bug 29472) kill $wgUseDynamicDates"