Deprecate $wgMasterWaitTimeout

Not only was this global unused, it also caused the LoadBalancer
to look for the wrong array key, which would always set
$mWaitTimeout to 10 in the conditional. There is also
a manual function LoadBalancer::waitTimeout() for this.

Change-Id: I4b409ef890bec5aac7f666a2ed3dff9ea8bc14cc
Merge "Deprecate $wgMasterWaitTimeout"