gdbstub: drop CPUEnv from gdb_exit()

gdb_exit() has never needed anything from env and I doubt we are going
to start now.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210108224256.2321-8-alex.bennee@linaro.org>