Revert "darwin.apple_sdk_11_0: add Security dependency on xpc"

This reverts commit 511f21df7c.

In apple_sdk_11_0, the xpc package contains only headers that are
already part of libsystem, so this change did nothing.

For Swift and `-fmodules`, this actually caused an error, because
there was now a duplicate module in the search path.

pkgs/os-specific/darwin/apple-sdk-11.0/frameworks.nix

@@ -159,7 +159,7 @@
   ScreenSaver                      = {};
   ScreenTime                       = {};
   ScriptingBridge                  = {};
-  Security                         = { inherit IOKit libDER xpc; };
+  Security                         = { inherit IOKit libDER; };
   SecurityFoundation               = { inherit Security; };
   SecurityInterface                = { inherit Security SecurityFoundation; };
   SensorKit                        = {};