target/riscv: remove CONFIG_TCG, as it is always TCG

target/riscv: remove CONFIG_TCG, as it is always TCG

for now only TCG is allowed as an accelerator for riscv,
so remove the CONFIG_TCG use.

Signed-off-by: Claudio Fontana <[email protected]>
Reviewed-by: Alistair Francis <[email protected]>
Reviewed-by: Alex Bennée <[email protected]>
Message-Id: <20210204163931[email protected]>
Signed-off-by: Richard Henderson <[email protected]>