riscv: remove RISC-V standalone linker script

Standalone applications do not require a separate linker script and can
use the default linker script of the compiler instead. Remove the RISC-V
standalone linker script.

Signed-off-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>