api: update for WSME 0.5b6 compliance

This makes use of the mandatory option of WSME, that now works, to
remove some of our custom validation code. This is needed for new
versions of WSME that do more validation on their own.

Fixes-Bug: #1240741

Change-Id: Icb66d17066b515bebf3f3a326d84e18cbfce01ef
Signed-off-by: Julien Danjou <julien@danjou.info>