stasis.c: Use correct topic name in stasis_topic_pool_delete_topic

When a topic is created for an object, its name is only
<object>:<uniqueid>
For example:
bridge:cb68b3a8-fce7-4738-8a17-d7847562f020

When a topic is added to a pool, its name has the pool's topic
name prepended.  For example:
bridge:all/bridge:cb68b3a8-fce7-4738-8a17-d7847562f020

The topic_pool_entry's name however, is only what was passed
in to stasis_topic_pool_get_topic which is
bridge:cb68b3a8-fce7-4738-8a17-d7847562f020
That's actually correct because the entry is qualified by the
pool that's in.

When you're ready to delete the entry from the pool, you retrieve
the tropic name from the object but since it now has the pool's
topic name prepended, it won't be found in the pool container.

Fix:

* Modified stasis_topic_pool_delete_topic() to skip past the
pool topic's name, if it was prepended to the topic name,
before searching the container for a pool entry.

ASTERISK-28633
Reported by: Joeran Vinzens

Change-Id: I4396aa69dd83e4ab84c5b91b39293cfdbcf483e6