Remove chromium-only flag that was added for a test we deleted long ago

See crbug.com/53408, https://crbug.com/513881
Reverts https://codereview.chromium.org/1253993004

Bug: 961769
Change-Id: I9e18947449562da668e1d842ae89461f3fbe2b14
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1714413Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680210}

extensions/common/feature_switch.cc

@@ -24,16 +24,13 @@ class CommonSwitches {
   CommonSwitches()
       : force_dev_mode_highlighting(switches::kForceDevModeHighlighting,
                                     FeatureSwitch::DEFAULT_DISABLED),
-        prompt_for_external_extensions(
-#if defined(CHROMIUM_BUILD)
-            switches::kPromptForExternalExtensions,
-#else
-            nullptr,
-#endif
+        // Intentionally no flag since turning this off outside of tests
+        // is a security risk.
+        prompt_for_external_extensions(nullptr,
 #if defined(OS_WIN) || defined(OS_MACOSX)
-            FeatureSwitch::DEFAULT_ENABLED),
+                                       FeatureSwitch::DEFAULT_ENABLED),
 #else
-            FeatureSwitch::DEFAULT_DISABLED),
+                                       FeatureSwitch::DEFAULT_DISABLED),
 #endif
         error_console(switches::kErrorConsole, FeatureSwitch::DEFAULT_ENABLED),
         enable_override_bookmarks_ui(switches::kEnableOverrideBookmarksUI,

extensions/common/switches.cc

@@ -66,12 +66,6 @@ const char kLoadApps[] = "load-apps";
 // Comma-separated list of paths to extensions to load at startup.
 const char kLoadExtension[] = "load-extension";
 
-#if defined(CHROMIUM_BUILD)
-// Should we prompt the user before allowing external extensions to install?
-// This flag is available on Chromium for testing purposes.
-const char kPromptForExternalExtensions[] = "prompt-for-external-extensions";
-#endif
-
 // Set the parameters for ExtensionURLLoaderThrottleBrowserTest.
 const char kSetExtensionThrottleTestParams[] =
     "set-extension-throttle-test-params";

extensions/common/switches.h

@@ -26,9 +26,6 @@ extern const char kForceDevModeHighlighting[];
 extern const char kForceEmptyCorbAllowlist[];
 extern const char kLoadApps[];
 extern const char kLoadExtension[];
-#if defined(CHROMIUM_BUILD)
-extern const char kPromptForExternalExtensions[];
-#endif
 extern const char kSetExtensionThrottleTestParams[];
 extern const char kShowComponentExtensionOptions[];
 extern const char kTraceAppSource[];