Remove evergreen test shard from evergreen-x64.json (#753)

The Evergreen tests aren't (yet) supported for LTS branches.

The checks are passing now because the Evergreen test cases were temporarily disabled in the Evergreen test suite some time ago and #566, which re-enabled them, never made it to 24.lts.1+. The intent is more clear if we remove the evergreen test shard from the actions config, though.

b/222720328

.github/config/evergreen-x64.json

@@ -2,7 +2,7 @@
   "docker_service": "build-linux-evergreen",
   "on_host_test": true,
   "bootloader": "linux-x64x11",
-  "on_host_test_shards": ["0", "1", "2", "3", "blackbox", "wpt", "evergreen"],
+  "on_host_test_shards": ["0", "1", "2", "3", "blackbox", "wpt"],
   "platforms": [
     "evergreen-x64",
     "evergreen-x64-sbversion-15",