Revert r21158 and r21159 -- clearly broken code.

Revert r21158 and r21159 -- clearly broken code.
wfGetClientIPfromXFF() uses undeclared variables -- it will never return a non-null value, but will in most cases trigger PHP notices about accessing unused variables.
It's also not documented as to what it does or how it's supposed to differ from the functions above it.
Please document, rewrite, and test this function if it is required.