Remove "common search program" internal feature

There is a (undocumented) configuration key that switches the search
for any handler to a preset program. However, there are different issues
with it:
- letting handlers to create their indexes as needed, but then not using
  them, makes no sense -- my blind guess is in that far past some distro
  used this to integrate with their own system, but I cannot find more
  hints about that
- the common program seems to behave like a CGI
- the internal implementation is synchronously in a buggy way (cfr
  bug #158282)
- it has not been tested nor used for a decade

Hence, just drop this implementation.
BUG: 158282