Remove x86image element test

Freedesktop-sdk have removed the elements this depends on, and so it will likely never hit the cache and need to rebuild way too much for a single test in the test suite. This will mean that (for now) there is no test of the x86image plugin, but given this plugin is relatively stable and probably soon to be deprecated, I think this shouldn't be too much of a loss to the gain of... working tests in CI.

Also as fdsdk uses the git_tag plugin and some others in here, changing the cache key of those will mean this test has to re build everything, which is no good also. If we had a cache server for this repo things would be a little better, but I think removing this test is to the benefit of all really.