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.

applications/utilities/mesh/generation/snappyHexMesh/snappyHexMesh.C

@@ -916,7 +916,7 @@ int main(int argc, char *argv[])
                         "decomposeParDict",
                         runTime.system(),
                         mesh,
-                        IOobject::MUST_READ_IF_MODIFIED,
+                        IOobject::MUST_READ,
                         IOobject::NO_WRITE
                     ),
                     decompDictFile

applications/utilities/parallelProcessing/decomposePar/decomposePar.C

@@ -347,7 +347,7 @@ int main(int argc, char *argv[])
                         runTime.time().system(),
                         regionDir,          // use region if non-standard
                         runTime,
-                        IOobject::MUST_READ_IF_MODIFIED,
+                        IOobject::MUST_READ,
                         IOobject::NO_WRITE,
                         false
                     ),

applications/utilities/preProcessing/mapFields/mapFields.C

@@ -69,7 +69,7 @@ int readNumProcs
                     dictName,
                     runTime.system(),
                     runTime,
-                    IOobject::MUST_READ_IF_MODIFIED,
+                    IOobject::MUST_READ,
                     IOobject::NO_WRITE,
                     false
                 ),

applications/utilities/surface/surfaceRedistributePar/surfaceRedistributePar.C

@@ -170,7 +170,7 @@ int main(int argc, char *argv[])
                     "decomposeParDict",
                     runTime.system(),
                     runTime,
-                    IOobject::MUST_READ_IF_MODIFIED,
+                    IOobject::MUST_READ,
                     IOobject::NO_WRITE
                 ),
                 decompDictFile

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
                         )
                     )