Revert of Move gn _run target generation into test().

testing/test.gni

Index: testing/test.gni
diff --git a/testing/test.gni b/testing/test.gni
index 1c2a927717f99bd565aaa2db1ed20d0919b2f263..acd1e1acab3408cca6403648d5efa88f4212caeb 100644
--- a/testing/test.gni
+++ b/testing/test.gni
@@ -186,8 +186,6 @@
       ]
     }
   } else {
-    main_target_name = target_name
-
     executable(target_name) {
       forward_variables_from(invoker, "*")
 
@@ -205,14 +203,5 @@
         "//build/win:default_exe_manifest",
       ]
     }
-
-    # TODO(GYP): Delete this after we've converted everything to GN.
-    # The _run targets exist only for compatibility with GYP.
-    group("${target_name}_run") {
-      testonly = true
-      deps = [
-        ":$main_target_name",
-      ]
-    }
   }
 }