initramfs: Remove compatibility warnings regarding CRYPTSETUP=[y|n].

The warning was added in 2:2.0.3-1 for the ‘cryptsetup’ package split
into ‘cryptsetup-run’ and ‘cryptsetup-initramfs’. With the recent
Buster release we can now move on: cryptsetup is always copied to the
initramfs when the ‘cryptsetup-initramfs’ package is installed.