svcrdma: Remove unused Read completion handlers

Clean up:

The generic RDMA R/W API conversion of svc_rdma_recvfrom replaced
the Register, Read, and Invalidate completion handlers. Remove the
old ones, which are no longer used.

These handlers shared some helper code with svc_rdma_wc_send. Fold
the wc_common helper back into the one remaining completion handler.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>