Use real(..,kind=CUSTOM_REAL) for type conversion.

There are several cases of:

    if (CUSTOM_REAL == SIZE_REAL) then
    ! do stuff
    result = sngl(...)
    else
    ! do stuff
    result = ...
    endif

which causes type conversion warnings for the latter half. It is much
simpler to use the kind argument to the real() intrinsic. This fixes the
warning and means that the code doesn't need to be written twice.
Use MPI_STATUS_IGNORE in MPI calls.

The status is not used, so this saves us an extra variable or two. As
suggested by @jedbrown in another PR for Specfem2D.
Remove explicitly imported but unused module vars.