math/deal.ii: unbreak by disabling the mumps option

deal.ii was built successfully when math/mumps wasn't installed.
Due to some recent rearrangements in dependencies math/mumps
is now installed and this triggered failure in deal.ii because
it expects a newer mumps.

What was done: the MUMPS option was disabled to fix build.

TODO: math/mumps, which is currently Code-Aster specific, should

be moved to math/mumps-code-aster, and math/mumps should
contain the latest version of mumps.