app/testpmd: fix duplicate exit

app/testpmd: fix duplicate exit

In interactive mode, when the "quit" command is issued, pmd_test_exit()
is being called twice, once through the "quit" command and the other
after termination of prompt.

Remove duplicated exit routine by removing call from "quit" command.

Steps to reproduce:
- Run testpmd in interactive mode.
- type "quit".

Fixes: af75078fece3 ("first public release")
Cc: stable@dpdk.org
Signed-off-by: Brian Archbold <brian.archbold@intel.com>
Acked-by: Bernard Iremonger <bernard.iremonger@intel.com>