callCabal2nix: Fix filtering for non-cleanSourceable sources.

What was here before wasn't correct anyway, and now it works in
restricted mode.

Fixes #35207

pkgs/build-support/safe-discard-string-context.nix

@@ -1,14 +0,0 @@
-# | Discard the context of a string while ensuring that expected path
-# validity invariants hold.
-#
-# This relies on import-from-derivation, but it is only useful in
-# contexts where the string is going to be used in an
-# import-from-derivation anyway.
-#
-# safeDiscardStringContext : String → String
-{ writeText }: s:
-  builtins.seq
-    (import (writeText
-               "discard.nix"
-               "${builtins.substring 0 0 s}null\n"))
-    (builtins.unsafeDiscardStringContext s)