lib/digsig: pkcs_1_v1_5_decode_emsa cleanup

Removed useless 'is_valid' variable in pkcs_1_v1_5_decode_emsa(),
which was inhereted from original code. Client now uses return value
to check for an error.

Signed-off-by: Dmitry Kasatkin <dmitry.kasatkin@intel.com>
Reviewed-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: James Morris <jmorris@namei.org>