Refactor: libcrmcommon: Drop redundant "Allowed values" from longdesc

For "select"-type options, the allowed values are already enumerated
separately in the <content> element. For the one non-"select"-type
option with an allowed-values string
(PCMK_OPT_CLUSTER_RECHECK_INTERVAL), we move the allowed values string
into longdesc itself.

This simplifies the add_desc() function without losing information.

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