Remove excessive locks from the function delete_htab_entry.
Function rtu_fd_commit is always guarded with shmem lock. The function
rtu_fd_commit calls rtu_hw_commit, which calls delete_htab_entry. By this
function wrs_shm_write with a parameter WRS_SHM_WRITE_BEGIN was called twice
before it was called twice with a parameter WRS_SHM_WRITE_END.

Signed-off-by: Adam Wujek <adam.wujek@cern.ch>