script: gen_isr_tables: remove unnecessary warning log

Remove a message that had a typo in it but, more importantly that was
not needed.
The CONFIG_ISR_TABLES_LOCAL_DECLARATION already selects EXPERIMENTAL so
a user is properly alerted about the consequences just like any other
EXPERIMENTAL features if they enable WARN_EXPERIMENTAL.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>