[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.
### Requirements for tests in various cubicweb/**/test directories. ###
## shared by several test folders
cubicweb-card
docutils
Twisted < 16.0.0
webtest
## cubicweb/test
Pygments
mock
#fyzz XXX pip install fails
cubicweb-file
cubicweb-localperms
cubicweb-tag
## cubicweb/devtools/test
flake8
## cubicweb/hooks/test
psycopg2
## cubicweb/pyramid/test
pyramid >= 1.5.0
waitress >= 0.8.9
wsgicors >= 0.3
pyramid_multiauth
## cubicweb/sobject/test
cubicweb-comment