[infra] Remove obsolete "global" key from the test_matrix.json

Follow-up to https://dart-review.googlesource.com/c/recipes/+/203620.

Change-Id: I179a1084212ec66cf7a74be8bcf81f15a4493d6f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/204120
Reviewed-by: Karl Klose <karlklose@google.com>
Commit-Queue: Alexander Thomas <athom@google.com>

tools/bots/README.md

@@ -12,7 +12,7 @@ infrastructure. Changes to the test matrix affect all builds that include them.
 
 The test matrix is a JSON document and consists of the `"filesets"` object, the
 `"configurations"` list, and the `"builder_configurations"` list as well as a
-`"global"` values object and a `"branches"` list.
+`"branches"` list.
 
 ### Filesets
 

tools/bots/test_matrix.json

@@ -1,8 +1,4 @@
 {
-  "global": {
-    "chrome": "81",
-    "firefox": "67"
-  },
   "branches": [
     "master"
   ],