ht://dig is basically unmantained for the past decade, and its
integration in khelpcenter has always been a sort of hack (execute
a CGI, and print its resulting HTML text).

Since Xapian is used as better solution, all of the htp://dig
supporting code can be removed completely.

CCBUG: 103266
CCBUG: 112958
BUG: 122437
BUG: 125276
BUG: 152671
BUG: 158633
BUG: 209415
BUG: 244091
BUG: 255384
BUG: 256397
BUG: 277464