Dockerfile: remove libbtrfs-dev dependency

Starting with [containerd@52f82ac] (containerd 1.7), this dependency is no
longer needed for building containerd.

[containerd@52f82ac]: 52f82acb7b

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Dockerfile

@@ -209,7 +209,6 @@ RUN --mount=type=cache,sharing=locked,id=moby-containerd-aptlib,target=/var/lib/
     --mount=type=cache,sharing=locked,id=moby-containerd-aptcache,target=/var/cache/apt \
         apt-get update && xx-apt-get install -y --no-install-recommends \
             gcc \
-            libbtrfs-dev \
             pkg-config
 ARG DOCKER_STATIC
 RUN --mount=from=containerd-src,src=/usr/src/containerd,rw \