Revert of Turn on hermetic toolchain for all corp machines.

build/toolchain/toolchain.gni

Index: build/toolchain/toolchain.gni
diff --git a/build/toolchain/toolchain.gni b/build/toolchain/toolchain.gni
index 4e49678ab920a3f69e03899038b947a3ef62526b..ca4bc2a7790e67dec020285ce78ede46f02bbdeb 100644
--- a/build/toolchain/toolchain.gni
+++ b/build/toolchain/toolchain.gni
@@ -49,15 +49,9 @@
   use_system_xcode = ""
 }
 
-if (use_system_xcode == "") {
-  if (target_os == "mac") {
-    _result =
-        exec_script("//build/mac/should_use_hermetic_xcode.py", [], "value")
-    use_system_xcode = _result == 0
-  }
-  if (target_os == "ios") {
-    use_system_xcode = true
-  }
+if ((is_mac || is_ios) && use_system_xcode == "") {
+  _result = getenv("FORCE_MAC_TOOLCHAIN")
+  use_system_xcode = _result == ""
 }
 
 # The path to the hermetic install of Xcode. Only relevant when