strip: remove experimental.prunestrip option

marmoute decided to drop it because it's "neither documented nor a good idea".
8 jobs for topic/default/prunestrip in 4 minutes and 38 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #125382
checks-py2

00:02:56

passed #125383
checks-py3

00:01:42

passed #125388
doc

00:00:43

manual #125381
allowed to fail manual
pytype
passed #125384
tests-py2-cext

00:04:23

passed #125385
tests-py2-pure

00:04:27

passed #125386
tests-py3-cext

00:04:28

passed #125387
tests-py3-pure

00:04:37