MAJOR: mailers: remove native mailers support

MAJOR: mailers: remove native mailers support

As mentioned in 2.8 announce on the mailing list [1] and on the wiki [2]
native mailers were deprecated and planned for removal in 3.3. Now is
the time to drop the legacy code for native mailers which is based on a
tcpcheck "hack" and cannot be maintained. Lua mailers should be used as
a drop in replacement. Indeed, "mailers" and associated config directives
are preserved because mailers config is exposed to Lua, which helps smoothing
the transition from native mailers to Lua based ones.

As a reminder, to keep mailers configuration working as before without
making changes to the config file, simply add the line below to the global
section:

       lua-load examples/lua/mailers.lua

mailers.lua script (provided in the git repository, adjust path as needed)
may be customized by users familiar with Lua, by default it emulates the
behavior of the native (now removed) mailers.

[1]: https://www.mail-archive.com/haproxy@formilux.org/msg43600.html
[2]: https://github.com/haproxy/wiki/wiki/Breaking-changes