Revert "Fix race condition of $XDG_CACHE_HOME/rply creation." (refs: #17359).

This reverts commit c957b5b8.

With the new approach (implemented two commits ago), a cache directory
is created for each i18nspector call. This means that we don't need to
treat the first file specifically (which wasn't working, depending on
which file that was).

As a result, the code can be simplified, getting back to the previous
“treat all files the same way” implementation.

slaves/lint_po

@@ -330,15 +330,8 @@ def main(logger) -> None:
             _check_po_file = functools.partial(check_po_file,
                                                extended=args.extended,
                                                i18nspector_env=i18nspector_env)
-
-            # As we start 10 i18nspector instances, we run into a race
-            # condition, as all try to create $XDG_CACHE_HOME/rply, that's why
-            # we check the file before the others. see #17359#note-14
-            iterator = iter(files)
-            chain = [(_check_po_file(next(iterator)),),
-                     pool.imap_unordered(_check_po_file,iterator, 10)]
-
-            for fname, issues in itertools.chain.from_iterable(chain):
+            for fname, issues in pool.imap_unordered(_check_po_file,
+                                                     files, 10):
                 if issues:
                     fine = False
                     # indent sub-issues: