refactor-864370: Merge setAssignment functions
refactor-864370: Merge setAssignment functions

Now we've got rid of the extra events, we can merge
requisition._assignmentChanged and the setAssignmentInternal function
(inside setAssignment) into a single simpler _setAssignmentInternal
function.

Signed-off-by: Joe Walker <7a17872fbb32c7d760c9a16ee3076daead11efcf@mozilla.com>

joewalker/gcli,joewalker/gcli,mozilla/gcli,mozilla/gcli,mozilla/gcli