Revert r64853 (add $wgLogAutocreatedAccounts to enable/disable account autocreation...

Revert r64853 (add $wgLogAutocreatedAccounts to enable/disable account autocreation logging, bug 19161). Per the *extremely lengthy* discussion that is still ongoing, this fails to solve the underlying problem and has undesired side-effects.

I don't know what the proper solution is (other than introducing user preferences, like bug 28369 suggests, is not the answer)...but the status quo of what we had is better than this half-assed solution that *nobody* likes.