fetchurl: remove unpaired call to `stopNest'

Fixes #27406.

Commit 5d4efb2c81 added an assertion to `stopNest'
which requires it be correctly paired with `startNest'. `fetchurl' calls
`stopNest', but never calls `startNest'; the former calls are removed.

pkgs/build-support/fetchurl/builder.sh

@@ -39,7 +39,6 @@ tryDownload() {
           curlexit=$?;
        fi
     done
-    stopNest
 }
 
 
@@ -51,7 +50,6 @@ finish() {
     fi
 
     runHook postFetch
-    stopNest
     exit 0
 }