GlobalFunctions: Remove deprecated wfCountDown() function from core

GlobalFunctions: Remove deprecated wfCountDown() function from core

The global function `wfCountDown()` was deprecated in 1.31 and a replacement
made available (\Maintenance::countdown()) in 1.31 as well. Currently, the few
usage in BSF have been cleaned up, hence can be removed.

Usage
=====

https://codesearch.wmflabs.org/search/?q=wfCountDown&i=nope&files=&repos=

Depends-On: Iae61d36899036ecdc0341feb3f1bcaa6b38dbf44
Depends-On: Id202148c6a6ceaadcefcf7a4ab8360edd9fe3b0b
Bug: T220656
Change-Id: Ic7460f1d35cb6f2eb8bf60694cdd4c55b2e001c0