powerpc/boot: remove unused function find_node_by_linuxphandle()

The last user of find_node_by_linuxphandle() was removed in v4.18-rc1
by commit 30f4bbe0

 ("powerpc/boot: Remove support for Marvell MPSC serial controller")
four years ago.  This function is no longer needed.

Signed-off-by: Frank Rowand <frank.rowand@sony.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220424184014.968274-1-frowand.list@gmail.com