tests: kernel: gen_isr_table: Re-enable warning

This warning doesn't appear to be needed anymore and CI passes with it
removed.

Signed-off-by: Tom Hughes <tomhughes@chromium.org>

Author: thughes

tests/kernel/gen_isr_table/src/main.c

@@ -410,9 +410,6 @@ static void *gen_isr_table_setup(void)
 {
 	TC_PRINT("IRQ configuration (total lines %d):\n", CONFIG_NUM_IRQS);
 
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wunused-label"
-
 	return NULL;
 }