refactor(3p/nix): Remove unused insert_or_assign

This function in never called, so let's just remove it

Paired-With: Luke Granger-Brown <git@lukegb.com>
Paired-With: Vincent Ambo <mail@tazj.in>
Change-Id: I79125866254d90dd0842bc86830d2103ac313cb6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1125
Tested-by: BuildkiteCI
Reviewed-by: lukegb <lukegb@tvl.fyi>
Reviewed-by: isomer <isomer@tvl.fyi>

third_party/nix/src/libexpr/attr-set.hh

@@ -80,9 +80,6 @@ class Bindings {
   // Insert, but do not replace, values in the attribute set.
   virtual void push_back(const Attr& attr) = 0;
 
-  // Insert a value, or replace an existing one.
-  virtual void insert_or_assign(Attr& attr) = 0;
-
   // Look up a specific element of the attribute set.
   virtual iterator find(const Symbol& name) = 0;