MDSMonitor.cc: remove cmd_getval for poolid

Remove useless check for invalid parameter for
"mds remove_data_pool", we have already the poolid
from the correct parameter 'poolname'.

Fix for:

CID 1251445 (#1 of 1): Unchecked return value (CHECKED_RETURN)
 check_return: Calling cmd_getval without checking return value
 (is done elsewhere 19 out of 22 times).

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>