Delete maintenance/purgeOldText.inc

The only function it contains (PurgeRedundantText()) was superseded by
Maintenance::purgeRedundantText() in r52006, yet the file had not been
deleted. I grepped core and extensions in Gerrit for the names of both
the function and the file and found no usage.

The purgeOldText.php maintenance script uses the "new" (since 1.16)
method and has been retained.

Change-Id: I40afbd264242eab7fadc8352380779074d61d7f8
Merge "Delete maintenance/purgeOldText.inc" into REL1_24