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

examples/lua/mailers.lua

@@ -3,7 +3,7 @@
 -- Provides a pure lua alternative to tcpcheck mailers.
 --
 -- To be loaded using "lua-load" from haproxy configuration to handle
--- email-alerts directly from lua and disable legacy tcpcheck implementation.
+-- email-alerts directly from lua
 
 local SYSLOG_LEVEL = {
 	["EMERG"] = 0,
@@ -364,10 +364,6 @@ local function srv_event_add(event, data)
 	mailers_track_server_events(data.reference)
 end
 
-
--- disable legacy email-alerts since email-alerts will be sent from lua directly
-core.disable_legacy_mailers()
-
 -- event subscriptions are purposely performed in an init function to prevent
 -- email alerts from being generated too early (when process is starting up)
 core.register_init(function()