spec: Delete old unused directory

The protocol spec used to live here, but it's now part of the regular
doc build. The PNG files are created as part of the doc build. Delete
the pre-generated versions.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Derek Foreman <derek.foreman.samsung@gmail.com>
1 job for master in 1 minute and 53 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #12968
build-native

00:01:53