Remove webrtc_root_additional_dependencies from all.gyp

It is no longer used.

BUG=
R=henrika@webrtc.org

Review URL: https://codereview.webrtc.org/1886973003 .

Cr-Commit-Position: refs/heads/master@{#12360}

all.gyp

@@ -10,7 +10,6 @@
   'variables': {
     'include_examples%': 1,
     'include_tests%': 1,
-    'webrtc_root_additional_dependencies': [],
   },
   'targets': [
     {
@@ -18,7 +17,6 @@
       'type': 'none',
       'dependencies': [
         'webrtc/webrtc.gyp:*',
-        '<@(webrtc_root_additional_dependencies)',
       ],
       'conditions': [
         ['include_examples==1', {