chore(3p/nix): Remove support for plugins

Plugins seem to not really be used anywhere (I can find one plugin
that's actually defined, and it doesn't seem very useful, especially
since we got rid of builtins.exec) and their presence is adding
additional complexity and potential sources of bugs to an already
unsteady refactor. At some point we may want to bring back
something *like* plugins, but their design will likely be different and
it will definitely be after we have a functioning Nix again.

Change-Id: I3bc40e55917f70bf260fbc208c1705e2e6a7c626
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1291
Tested-by: BuildkiteCI
Reviewed-by: Alyssa Ross <hi@alyssa.is>
Reviewed-by: isomer <isomer@tvl.fyi>

third_party/nix/src/nix-copy-closure/nix-copy-closure.cc

@@ -48,8 +48,6 @@ static int _main(int argc, char** argv) {
           return true;
         });
 
-    initPlugins();
-
     if (sshHost.empty()) {
       throw UsageError("no host name specified");
     }