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>