powerpc: remove orphan systbl_chk.sh

arch/powerpc/kernel/systbl_chk.sh has not been referenced since commit
ab66dcc7 ("powerpc: generate uapi header and system call table
files"). Remove it.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220929032120.3592593-1-npiggin@gmail.com