x86: gen_idt.py: Simplify test with 'not in'

Getting slightly subjective, but fixes this pylint warning:

    arch/x86/gen_idt.py:281:11: R1714: Consider merging these
    comparisons with "in" to 'handler not in (spur_code, spur_nocode)'
    (consider-using-in)

Getting rid of pylint warnings for a CI check. I could disable any
controversial ones (it's already a list of warnings to enable anyway).

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>