Refactor: liblrmd: Explicitly ignore attrd flush command

PCMK__ATTRD_CMD_FLUSH (formerly ATTRD_OP_FLUSH) was removed in Pacemaker
2.0.0. The constant is maintained and the command accepted for
compatibility with older peers, but we should ignore it if we receive
it. This commit makes our intention explicit.

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