SUNRPC: Rename .encrypt_v2 and .decrypt_v2 methods

Clean up: there is now only one encrypt and only one decrypt method,
thus there is no longer a need for the v2-suffixed method names.

Tested-by: Scott Mayhew <smayhew@redhat.com>
Reviewed-by: Simo Sorce <simo@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>