SUNRPC: Remove ->encrypt and ->decrypt methods from struct gss_krb5_enctype

Clean up: ->encrypt is set to only one value. Replace the two
remaining call sites with direct calls to krb5_encrypt().

There have never been any call sites for the ->decrypt() method.

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