Remove long shipped CSS Environment variable setting.

The feature first shipped two stable releases ago in M69.

Bug=848890

Change-Id: Id969cf348d9523ec50803d7feadec7071b652730
Reviewed-on: https://chromium-review.googlesource.com/c/1323770Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Becca Hughes <beccahughes@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606161}

third_party/blink/renderer/core/css/parser/css_variable_parser.cc

@@ -36,8 +36,6 @@ bool ClassifyBlock(CSSParserTokenRange range,
           has_references = true;
           continue;
         case CSSValueEnv:
-          if (!RuntimeEnabledFeatures::CSSEnvironmentVariablesEnabled())
-            return false;
           if (!IsValidEnvVariableReference(range.ConsumeBlock(),
                                            skip_variables))
             return false;  // Invalid reference.

third_party/blink/renderer/core/css/resolver/css_variable_resolver_test.cc

@@ -40,7 +40,6 @@ class CSSVariableResolverTest : public PageTestBase {
   void SetUp() override {
     PageTestBase::SetUp();
 
-    RuntimeEnabledFeatures::SetCSSEnvironmentVariablesEnabled(true);
     GetStyleEngine().EnsureEnvironmentVariables().SetVariable("test", "red");
   }
 

third_party/blink/renderer/core/css/style_environment_variables_test.cc

@@ -41,8 +41,6 @@ class StyleEnvironmentVariablesTest : public PageTestBase {
   void SetUp() override {
     PageTestBase::SetUp();
 
-    RuntimeEnabledFeatures::SetCSSEnvironmentVariablesEnabled(true);
-
     // Needed for RecordUseCounter_IgnoreMediaControls.
     RuntimeEnabledFeatures::SetModernMediaControlsEnabled(true);
   }

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -249,10 +249,6 @@
       name: "CSSBackdropFilter",
       status: "experimental",
     },
-    {
-      name: "CSSEnvironmentVariables",
-      status: "stable",
-    },
     {
       name: "CSSFocusVisible",
       status: "experimental",