Remove unused kms plugin

Remove unused kms plugin

The old kms plugin has been replaced by EGLFS's KMS backend. The EGLFS
backend is smaller and has more features.

The kms plugin cannot be built automatically since Qt 5.5 and is
therefore also not tested for changes in QPA API by the CI. Therefore
removing the plugin from the tree should not be a regression from the
previous release.

Change-Id: Icb9ea8c4a5d6421397a2e3f2e2195fb6bfad2293
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>