Refactor: tools: Best practices in crm_shadow.c:shadow_teardown()

Remove redundant name argument (was always options.instance), add
doxygen, avoid strstr() for prompt comparison (it should be a full match
anyway), avoid overlong line. Tear down only if CIB_shadow matches the
given instance.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>