libbpf-tools: remove ext4dist

libbpf-tools: remove ext4dist

In #3441, we introduce a new libbpf tools named fsdist, which is
built on the idea by @anakryiko and previous work by @ethercflow.
fsdist extends ext4dist to support multiple file systems in a flexable
way. Now we can replace ext4dist and treat it as an alias to fsdist.
This commit removes ext4dist and replaces it with a symlink to fsdist.

References:
    #3430, #3436

Signed-off-by: Hengqi Chen <chenhengqi@outlook.com>