kernel: remove init_abort from _static_thread_data

Commit d823f88e3b34 ("kernel: move _static_thread_data to ROM") intended to
remove init_abort member from `struct _static_thread_data`, but instead it
just removed it from Z_THREAD_INITIALIZER() initializer.

Remove init_abort, which is a leftover.

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>
diff --git a/include/zephyr/kernel.h b/include/zephyr/kernel.h
index c71e475..c60edd1 100644
--- a/include/zephyr/kernel.h
+++ b/include/zephyr/kernel.h
@@ -660,7 +660,6 @@
 	int init_prio;
 	uint32_t init_options;
 	int32_t init_delay;
-	void (*init_abort)(void);
 	const char *init_name;
 };