drop compatibility shims

[totaam]

Now that xpra v5 and later is the only server version we have to be compatible with, we can remove a number of workarounds and compatibility shims. See:

• 259

• https://github.com/Xpra-org/xpra/issues/3592
• https://github.com/Xpra-org/xpra/issues/3812
• https://github.com/Xpra-org/xpra/issues/1942

ie:

• we no longer need to support rencode or bencode
• remove zlib etc.

[totaam]

See also:

• simplify packet encoding: 44001653288cf7f4b77492153eb8390884205f8e
• remove legacy encoders: 7ddf66abd23b08fbac31c28ea2f2c2224cf00f7c