chore: Remove //overrides from depot

This mechanism wasn't actually used for anything other than a minor
external compatibility thing for buildGo.nix, which can also be solved
with a function parameter.

This breaks //fun/gemma because it means that the elmPackages used to
build it are no longer reachable from depot. We'll sort this out later.

Change-Id: I1bf2240435e869cdc4e99bdd1a138fdd2e76f96e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2646
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

default.nix

@@ -71,6 +71,7 @@ let
            (node.meta.targets or []))
     else [];
 in fix(self: {
+  __readTree = [];
   config = config self;
 
   # Elevate 'lib' from nixpkgs
@@ -100,10 +101,4 @@ in fix(self: {
 
 # Add local packages as structured by readTree
 // (localPkgs (readTree' self.config))
-
-# Load overrides into the top-level.
-#
-# This can be used to move things from third_party into the top-level, too (such
-# as `lib`).
-// (readTree' { depot = self; pkgs = self.third_party; }) ./overrides
 )