cpu-exec: remove unnecessary check of cpu->exit_request

The cpu->exit_request check in cpu_loop_exec_tb is unnecessary,
because cpu->tcg_exit_req is always set after cpu->exit_request.
So let the TB exit and we will pick up the exit request later
in cpu_handle_interrupt.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
