target/riscv: remove cfg.ext_g setup from rv64_thead_c906_cpu_init()

target/riscv: remove cfg.ext_g setup from rv64_thead_c906_cpu_init()

This CPU is enabling G via cfg.ext_g and, at the same time, setting
IMAFD in set_misa() and cfg.ext_icsr.

riscv_cpu_validate_set_extensions() is already doing that, so there's no
need for cpu_init() setups to worry about setting G and its extensions.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Weiwei Li <liweiwei@iscas.ac.cn>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230406180351.570807-19-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>