kernel: remove deprecate wording on arch_kernel_init()

The wording on deprecating arch_kernel_init() in favor of prep_c()
has never been materialized. Various architectures are using it to
perform initialization. So remove the wording.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>

kernel/include/kernel_arch_interface.h

@@ -562,9 +562,6 @@ int arch_printk_char_out(int c);
  *
  * This function is invoked near the top of _Cstart, for additional
  * architecture-specific setup before the rest of the kernel is brought up.
- *
- * TODO: Deprecate, most arches are using a prep_c() function to do the same
- * thing in a simpler way
  */
 static inline void arch_kernel_init(void);