Revert "github: filters: build x86-64 target on Dockerfile change"

This reverts commit c418b7bb.

Our CI builds do not use the Docker image, so adding the Dockerfile to
the filter does not make sense. In addition, the entry was missing in
generate-target-filters.py, so it would have been removed when the
filters were regenerated.

.github/filters.yml

@@ -222,7 +222,6 @@
     "targets/targets.mk",
     "targets/x86.inc",
     "contrib/ci/minimal-site/**",
-    "contrib/docker/Dockerfile",
     "package/**"
   ],
   "bcm27xx-bcm2710": [