bpo.config.const: drop redundant kernel config for msm8916 devices
With the introduction of rproc selection toggle [1] the kernel variants were dropped. The "modem" mode is the default now for all images that are built with bpo and kernel selection section is a no-op.
Drop the kernel selecton across all msm8916 devices.
[1] https://gitlab.com/postmarketOS/pmaports/-/merge_requests/2632
cc: @Minecrell