Remove unused and broken escort energy center check

detect_escort_goal_accomplished is passed -4 for energy center and an
object index otherwise.  Using the index to search segments is wrong.
Using a side number to search other segments is also wrong.  Delete the
whole check, since it is broken and useless.
Simplify Buddy_objnum calculation
Simplify search for guide bot in mine
Simplify search for guide bot in escort menu
Handle ESCORT_GOAL_UNSPECIFIED in say_escort_goal