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/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
                     ),