riscv: vmlinux-xip.lds.S: remove .alternative section

ALTERNATIVE mechanism can't work on XIP, and this is also reflected by
below Kconfig dependency:

RISCV_ALTERNATIVE
	...
	depends on !XIP_KERNEL
	...

So there's no .alternative section at all for XIP case, remove it.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Guo Ren <guoren@kernel.org>
Tested-by: Nick Desaulniers <ndesaulniers@google.com> # build
Link: https://lore.kernel.org/r/20230523165502.2592-3-jszhang@kernel.org


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>