Remove the ht://dig support

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
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