nios2: exceptions: remove remaining muldiv references

We are not going to handle unimplemented math instruction
exceptions at runtime. Remove remaining comments and exports
related to this. We don't need to leave a gap in the exception
stack frame for it either.

Change-Id: I4f1f3980a0e43bbf6f2f7488a9182f7acb06be05
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>