msg: drop unnecessary polling_stop()

msg: drop unnecessary polling_stop()

Drop the unnecessary polling_stop() in the ~RDMAStack(). The RDMADispatcher object contained in
RDMAStack will get distroyed when RDMAStack gets distroyed. Thus ~RDMADispatcher() will be
called automatically and executes the polling_stop().

Fixes:
The below coverity scan reports.
1. CID 1416590:  Memory - corruptions
2. CID 1416593:  Memory - illegal accesses
3. CID 1416595:  Memory - corruptions
4. CID 1416597:  Memory - illegal accesses

Signed-off-by: Jos Collin <jcollin@redhat.com>