IB/qib: Remove unused declarations in header file

IB/qib: Remove unused declarations in header file

The definition of qib_rc_rnr_retry() has been removed since
commit b4238e70579c ("IB/qib: Use new rdmavt timers"). Also, the definition
of mr_rcu_callback() has been remove since commit 7c2e11fe2dbe ("IB/qib:
Remove qp and mr functionality from qib"). So, let's remove the unused
declartions.

Signed-off-by: Zhang Zekun <zhangzekun11@huawei.com>
Link: https://patch.msgid.link/20240909121408.80079-3-zhangzekun11@huawei.com
Signed-off-by: Leon Romanovsky <leon@kernel.org>