Move //mojo/shell to //shell

//shell is unambiguous in this repository and shorter.

R=ben@chromium.org

Review URL: https://codereview.chromium.org/775343004
diff --git a/BUILD.gn b/BUILD.gn
index 33384ff..8a57ce3 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -9,12 +9,9 @@
     "//examples",
     "//mojo",
     "//services",
+    "//shell",
   ]
 
-  if (!is_win) {
-    deps += [ "//sky" ]
-  }
-
   if (!is_android && !is_win) {
     deps += [ "//tools/xdisplaycheck" ]
   }