scsi: target: Remove unused se_tmr_req_cache declaration

se_tmr_req_cache has been removed since commit c8e31f26 ("target: Add
SCF_SCSI_TMR_CDB usage and drop se_tmr_req_cache").

Remove extern.

Link: https://lore.kernel.org/r/20220913023722.547249-3-cuigaosheng1@huawei.com


Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>