The codec `strip` option used to remove codecs not only from the
destination (offer to) monologue, but also from the source (offered by)
monologue. Change this to only affect codecs on the destination
(offering to) side. This fixes a problem of not being able to have
different `strip` options set for different branches.
As a side effect, this breaks existing use cases of using `strip` to
ignore codecs on the source monologue side. This functionality is
restored by adding `codec-ignore` in the following commit.
Change-Id: I022748fffc29caa3a8c96f514b2021f152e43686