BUG: surfaceWriter::expire() should not affect the expected nFields() count

- the writer nFields() is a separate type of accounting (currently
  only needed by VTK legacy) and is independent of the geometry.

src/surfMesh/writers/surfaceWriter.C

@@ -384,9 +384,11 @@ bool Foam::surfaceWriter::expire()
 
     upToDate_ = false;
     wroteGeom_ = false;
-    nFields_ = 0;
     merged_.clear();
 
+    // Field count (nFields_) is a different type of accounting
+    // and is unaffected by geometry changes
+
     return changed;
 }