ptxed: drop default exec mode options

Ptxed offers to set the default execution mode. This is unnecessary since
the execution mode needs to be provided in PSB+.

Drop the feature so we don't have to maintain it.

Change-Id: I16ae32db3179ba98df48afae389f4d5c9d6f168b
Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>