Partial revert of r36804:

Partial revert of r36804:
Removing the wfSharedTable function.
The function uses a $wgExternalSharedDB which is not part of Core, and has not been defined in default settings. The variable itself has no apparent purpose and is likely to make the existing use of $wgSharedDB even more complex and prone to breakage.

The function wfSharedTable itself, appears more like a Wikia made function to support transition of 1.12 wiki in which $wgSharedTables did not exist over to 1.13 in which it does exist.
If this is the actual point of it, then it serves no point in being added to core. 1.13 already has $wgSharedTables and it serves better to append to that, than to use wfSharedTable. This is something that extensions should be defining themselves if they need it for transition from old 1.12 wiki to 1.13.