target/riscv: Remove sideleg and sedeleg

sideleg and sedeleg csrs are not part of riscv isa spec
anymore, these csrs were part of N extension which
is removed from the riscv isa specification.

These commits removed all traces of these csrs from
riscv spec (https://github.com/riscv/riscv-isa-manual

) -

commit f8d27f805b65 ("Remove or downgrade more references to N extension (#674)")
commit b6cade07034d ("Remove N extension chapter for now")

Signed-off-by: Rahul Pathak <rpathak@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20220824145255.400040-1-rpathak@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>