hwdef: ARKV6X: enable BIDIR on default target, remove ARKV6X-bdshot

[dakejahl]

Summary

Enable bi-directional DShot on the default ARKV6X target and remove the separate ARKV6X-bdshot board.

Classification & Testing (check all that apply and add your own)

• Checked by a human programmer
• Non-functional change
• No-binary change
• Infrastructure change (e.g. unit tests, helper scripts)
• Automated test(s) verify changes (e.g. unit test, autotest)
• Tested manually, description below (e.g. SITL)
• Tested on hardware
• Logs attached
• Logs available on request

Description

Per discussion with @andyp1per: on H7, BIDIR is effectively free when complementary timer channels are not required for other functions (e.g. buzzer). ARKV6X uses TIM14 for the buzzer, so that caveat does not apply.

AI-assisted: draft implementation and PR text prepared with AI tools; human-reviewed before submit.