target/riscv: Remove some unused macros

target/riscv: Remove some unused macros

Since commit 1a9540d1f1a
("target/riscv: Drop support for ISA spec version 1.09.1")
these definitions are unused, remove them.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Message-id: f4d8a7a035f39c0a35d44c1e371c5c99cc2fa15a.1634531504.git.alistair.francis@wdc.com