removeOldManualUserPages: Fix path issues when trying to determine $IP.

In the typical directory layout: core/extensions/GlobalCssJs, $IP
should be "core", meaning we need to go up two directories, or
"/../..". I probably copied this out of CentralAuth which stores
maint scripts in a subdirectory inside the extension, requiring a
third "/..".

This wasn't noticed until now because presumably everywhere this script
has been run (jenkins, vagrant, WMF cluster) defines the "MW_INSTALL_PATH"
environment variable.

Change-Id: I766c77d6607f0ddb1e66a7cfaf0972fea1163360
Merge "removeOldManualUserPages: Fix path issues when trying to determine $IP."