Don't find_neighbors when serializing mesh if mesh indicates not to
Don't find_neighbors when serializing mesh if mesh indicates not to

This fixes currently failing tests on idaholab/moose#14456. When
writing to exodus we serialize the mesh, and this traditionally
has always called to `UnstructuredMesh::find_neighbors`. However,
we may not want to run `find_neighbors` per idaholab/moose#14456.

libMesh/libmesh,libMesh/libmesh,libMesh/libmesh,dschwen/libmesh,BalticPinguin/libmesh,jwpeterson/libmesh,jwpeterson/libmesh,dschwen/libmesh,libMesh/libmesh,jwpeterson/libmesh,libMesh/libmesh,jwpeterson/libmesh,roystgnr/libmesh,BalticPinguin/libmesh,dschwen/libmesh,dschwen/libmesh,BalticPinguin/libmesh,libMesh/libmesh,BalticPinguin/libmesh,BalticPinguin/libmesh,jwpeterson/libmesh,BalticPinguin/libmesh,dschwen/libmesh,BalticPinguin/libmesh,roystgnr/libmesh,roystgnr/libmesh,dschwen/libmesh,roystgnr/libmesh,roystgnr/libmesh,BalticPinguin/libmesh,dschwen/libmesh,libMesh/libmesh,roystgnr/libmesh,libMesh/libmesh,jwpeterson/libmesh,dschwen/libmesh,roystgnr/libmesh,jwpeterson/libmesh,jwpeterson/libmesh,roystgnr/libmesh