Drop the "light-level" Media Queries at-rule (#6706)

This change completely drops the "light-level" Media Queries at-rule
from BCD. It was never implemented in any browser engines, and has now
been removed from the Media Queries spec:

* https://drafts.csswg.org/mediaqueries-5/#changes-since-2020-07-15
  > Remove the "light-level" media feature as it is redundant
  > with "prefers-contrast" and "prefers-color-scheme"; add examples of how these
  > media features may be automatically inferred by the user agent based
  > on the same factors "light-level" was expected to respond to.
* w3c/csswg-drafts@f5b663c
* w3c/csswg-drafts#5359

I also filed mdn/sprints#3695 to delete the
https://developer.mozilla.org/en-US/docs/Web/CSS/@media/light-level
MDN article.

css/at-rules/media.json

@@ -854,55 +854,6 @@
             }
           }
         },
-        "light-level": {
-          "__compat": {
-            "description": "<code>light-level</code> media feature",
-            "mdn_url": "https://developer.mozilla.org/docs/Web/CSS/@media/light-level",
-            "support": {
-              "chrome": {
-                "version_added": false
-              },
-              "chrome_android": {
-                "version_added": false
-              },
-              "edge": {
-                "version_added": false
-              },
-              "firefox": {
-                "version_added": false
-              },
-              "firefox_android": {
-                "version_added": false
-              },
-              "ie": {
-                "version_added": false
-              },
-              "opera": {
-                "version_added": false
-              },
-              "opera_android": {
-                "version_added": false
-              },
-              "safari": {
-                "version_added": false
-              },
-              "safari_ios": {
-                "version_added": false
-              },
-              "samsunginternet_android": {
-                "version_added": false
-              },
-              "webview_android": {
-                "version_added": false
-              }
-            },
-            "status": {
-              "experimental": false,
-              "standard_track": true,
-              "deprecated": false
-            }
-          }
-        },
         "media_features": {
           "__compat": {
             "description": "Media feature expressions",