openrisc: Remove unused tlb_init function

When compiling with W=1 enabling -Wmissing-prototypes the compiler
warns:

  arch/openrisc/mm/tlb.c:188:13: error: no previous prototype for 'tlb_init' [-Werror=missing-prototypes]

This function is not implemented or used so remove it.
Reported-by: Arnd Bergmann <arnd@arndb.de>
Closes: https://lore.kernel.org/linux-kernel/20230810141947.1236730-17-arnd@kernel.org/Signed-off-by: Stafford Horne <shorne@gmail.com>

arch/openrisc/mm/init.c

@@ -123,8 +123,6 @@ static void __init map_ram(void)
 
 void __init paging_init(void)
 {
-	extern void tlb_init(void);
-
 	int i;
 
 	printk(KERN_INFO "Setting up paging and PTEs.\n");

arch/openrisc/mm/tlb.c

@@ -182,12 +182,3 @@ void destroy_context(struct mm_struct *mm)
 	flush_tlb_mm(mm);
 
 }
-
-/* called once during VM initialization, from init.c */
-
-void __init tlb_init(void)
-{
-	/* Do nothing... */
-	/* invalidate the entire TLB */
-	/* flush_tlb_all(); */
-}