[lldb] [NFC] Remove min pkt size for compression setting (#81075)

debugserver will not compress small packets; the overhead of the
compression header makes this useful for larger packets. I default to
384 bytes in debugserver, and added an option for lldb to request a
different cutoff. This option has never been used in lldb in the past
nine years, so I don't think there's any point to keeping it around.