STYLE: use MUST_READ consistently for decomposeParDict

- there was a slight mix of MUST_READ and MUST_READ_IF_MODIFIED
  but with no obvious code to handle runtime modified values
  of the decomposition, or how this works with alternative
  dictionaries.

src/parallel/distributed/distributedTriSurfaceMesh/distributedTriSurfaceMesh.C

@@ -847,7 +847,7 @@ Foam::distributedTriSurfaceMesh::independentlyDistributedBbs
                             "decomposeParDict",
                             searchableSurface::time().system(),
                             searchableSurface::time(),
-                            IOobject::MUST_READ_IF_MODIFIED,
+                            IOobject::MUST_READ,
                             IOobject::NO_WRITE
                         )
                     )