stubtest: Improve error message for __all__-related errors (#14362)

This error is *only* emitted if `__all__` is included in the stub, so
the 'if present' clause is unnecessary

Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>

mypy/stubtest.py

@@ -267,7 +267,7 @@ def _verify_exported_names(
         object_path + ["__all__"],
         (
             "names exported from the stub do not correspond to the names exported at runtime. "
-            "This is probably due to things being missing from the stub, or if present, an inaccurate `__all__` in the stub"
+            "This is probably due to things being missing from the stub or an inaccurate `__all__` in the stub"
         ),
         # Pass in MISSING instead of the stub and runtime objects, as the line numbers aren't very
         # relevant here, and it makes for a prettier error message