SUNRPC: Remove EXPORT_SYMBOL_GPL for svc_process_bc()

svc_process_bc(), previously known as bc_svc_process(), was
added in commit 4d6bbb62 ("nfs41: Backchannel bc_svc_process()")
but there has never been a call site outside of the sunrpc.ko
module.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>