vscode: simplify helper scripts

This simplifies the following commit.

- Remove `--` from `rm` because `$tmpDir` is guaranteed to never start
  with a `-`.
  We also don't use this pattern when removing tmpDirs at other places
  in nixpkgs.
- Remove `unset tmpDir` because the variable `tmpDir` is never
  accessed after the exit handler is run.

pkgs/applications/editors/vscode/extensions/_maintainers/update-bin-srcs-lib.sh

@@ -37,9 +37,8 @@ prefetchExtensionUnpacked() {
   1>&2 echo "zipStorePath='$zipStorePath'"
 
   function rm_tmpdir() {
-    1>&2 printf "rm -rf -- %q\n" "$tmpDir"
-    rm -rf -- "$tmpDir"
-    unset tmpDir
+    1>&2 printf "rm -rf %q\n" "$tmpDir"
+    rm -rf "$tmpDir"
     trap - INT TERM HUP EXIT
   }
   function make_trapped_tmpdir() {

pkgs/applications/editors/vscode/extensions/cpptools/update_helper.sh

@@ -44,9 +44,7 @@ extStoreName="${extPublisher}-${extName}"
 
 
 function rm_tmpdir() {
-  #echo "Removing \`tmpDir='$tmpDir'\`"
-  rm -rf -- "$tmpDir"
-  unset tmpDir
+  rm -rf "$tmpDir"
   trap - INT TERM HUP EXIT
 }
 function make_trapped_tmpdir() {