Remove unused BUILD flag

The flag usage has been removed from devtools-frontend [1].

[1]: https://crrev.com/c/devtools/devtools-frontend/+/1942288

Change-Id: I1436bc8be3258c84a86be9d86df9434647fca180
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1942336Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Tim van der Lippe <tvanderlippe@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720594}

third_party/blink/public/BUILD.gn

@@ -489,7 +489,6 @@ source_set("blink_headers") {
   ]
 }
 
-assert(external_devtools_frontend)
 devtools_frontend_path = "//third_party/devtools-frontend/src"
 
 group("blink_devtools_frontend_resources") {

third_party/blink/public/public_features.gni

@@ -10,10 +10,6 @@ declare_args() {
   # resources.pak. It is still possible to load JS files from disk by passing
   # --debug-devtools cmdline switch.
   debug_devtools = false
-
-  # If external_devtools_frontend is set to true, use DevTools frontend that is
-  # pulled as external dependency.
-  external_devtools_frontend = true
 }
 
 # Unhandled Tap enable means Contextual Search aka Tap to Search.