Fix domain decomposition overwriting muscl_order

The IGR conditional block unconditionally reset recon_order to
weno_order in its else branch, overwriting the muscl_order that
was correctly set by the recon_type check above. Remove the else
branch so the original recon_order is preserved when IGR is inactive.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: sbryngelson

src/common/m_mpi_common.fpp

@@ -1146,8 +1146,6 @@ contains
 
         if (igr) then
             recon_order = igr_order
-        else
-            recon_order = weno_order
         end if
 
         ! 3D Cartesian Processor Topology