guru: drop guru support

Guru has been removed from golang.org/x/tools/cmd/guru as described in
https://docs.google.com/document/d/1_Y9xCEMj5S-7rv2ooHpZNH15JgRT5iM742gJkw5LtmQ/edit#heading=h.ojv16z1d1gas.

Author: bhcleek