revert(nix/utils): onlyDrvPath to get the drvPath w/o the outputs

This reverts commit 6813598c17.

Reason for revert: This function is a re-implementation of
builtins.unsafeDiscardOutputDependency which I missed at the time.

Change-Id: I5bb52bfd5e8d51defaf90ee795b0fe99be84f6db
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7265
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

nix/utils/default.nix

@@ -43,21 +43,6 @@ let
     else builtins.throw "Don't know how to get (base)name of "
       + lib.generators.toPretty { } p;
 
-  /* Retrieves the drvPath attribute from a given derivation, but ensures that
-     the resulting string only depends on the `.drv` file in the nix store and
-     not on its realised outputs as well.
-
-     Type: drv -> string
-  */
-  onlyDrvPath = drv:
-    let
-      inherit (drv) drvPath;
-      unsafeDrvPath = builtins.unsafeDiscardStringContext drvPath;
-    in
-    builtins.appendContext unsafeDrvPath {
-      ${unsafeDrvPath} = { path = true; };
-    };
-
   /* Query the type of a path exposing the same information as would be by
      `builtins.readDir`, but for a single, specific target path.
 
@@ -167,7 +152,6 @@ in
 {
   inherit
     storePathName
-    onlyDrvPath
     pathType
     isDirectory
     isRegularFile