min should match always on for /search

Removes the need for frontend to specify it wants the feature.

This will mean:
  * service-manual will get the benefit of min should match (fewer, more
    relevant results)
  * searches via Content API will get the benefit without end-users needing to
    specifically request it
  * frontend doesn't need to specify it wants the feature

Arbitrary min should match rules were supported, but not used. I felt it wasn't worth keeping if it was unused, especially as it wasn't really documented and it was fairly complex.