Revert "core & res_pjsip: Improve topology change handling."

This reverts commit 315eb551.

Over the past year, we've had several reports of "topology storms"
occurring where 2 external facing channels connected by one or more
local channels and bridges will get themselves in a state where
they continually send each other topology change requests.  This
usually manifests itself in no-audio calls and a flood of
"Exceptionally long queue length" messages.  It appears that this
commit is the cause so we're reverting it for now until we can
determine a more appropriate solution.

Resolves: #530
(cherry picked from commit c31cd32b8296c675f66be7a4d39e89b45c3aec2c)