scripts: build: gen_isr_tables: Cleanup access to ".intList"

This commit removes the needs or generating isrList.bin temporary file.
Now gen_isr_tables.py script access the required section directly in
elf file, that was accessed by the script anyway.
It simplifies the building removing one step.

Signed-off-by: Radosław Koppel <radoslaw.koppel@nordicsemi.no>