gen_isr_tables: Cleanup IRQ vector table generation

Under no circumstances the generated IRQ vector table can and should
contain NULL values. This is correctly enforced at generation time by
the gen_isr_tables.py script making the existence of the ISR_WRAPPER
define useless.

The enforced behaviour is:
- When the ISR software table exists defaults to _isr_wrapper
- Otherwise defaults to z_irq_spurious

Signed-off-by: Carlo Caione <ccaione@baylibre.com>