powerpc: delete empty config entry for PPC_86xx

Drop an unused and empty "config" entry for PPC_86xx.
It has no keyword entries under it and the following line's
"menuconfig" for the same Kconfig symbol is what kconfig uses.

Fixes: d8267c1a ("powerpc: Add 82xx/83xx/86xx to 6xx Multiplatform")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230429043503.19385-1-rdunlap@infradead.org

arch/powerpc/platforms/86xx/Kconfig

 # SPDX-License-Identifier: GPL-2.0
-config PPC_86xx
 menuconfig PPC_86xx
 	bool "86xx-based boards"
 	depends on PPC_BOOK3S_32