[web] Clear "pending_others" session key after all relations got processed
The RelationField appends data to "pending_others" key while processing its
posted information, but this key may already have been removed by the edit
controller. So remove the key from session data only when all "others"
relations have been processed using a while loop.
A test case would be nice, but I could not build one easily unfortunately.
Closes #4354551.