sandbox: Drop CONFIG_SYS_TIMER_RATE

sandbox: Drop CONFIG_SYS_TIMER_RATE

This is not used by sandbox since it uses driver model for the timer.

Drop it.

Also update the tools_only build to avoid build errors, since it does
actually build U-Boot too. Enable DM so we can use CONFIG_TIMER,
disable EFI_LOADER to avoid an error about board_quiesce_devices() and
disable NET to avoid having to define CONFIG_AVB_BUF_ADDR

Signed-off-by: Simon Glass <sjg@chromium.org>