release: remove arm/armv6 RPI-B configuration file

The arm/armv6 RPI-B images are deprecated in 15 and 14.
An MFC to stable/14 will follow.

Approved by: re (cperciva)
Sponsored by: GoFundMe https://www.gofundme.com/f/gjbbsd
Sponsored by: PayPal https://paypal.me/gjbbsd

(cherry picked from commit 82c57e2a75c7134b70eb2138ff6bbc6e55bef130)