perftune.py: deprecate `mode`

[vladzcloudius]

HEAD: 5b34dfe230a8c0d816ebf5260e88af92e80f9bdd

Description We want to stop using --mode parameter and switch to irq_cpu_mask instead. We should continue supporting --mode/mode as an input for backward compatibility but stop generating it in the output of the --dump-options-file.