Deprecate libbeat processor support

[axw]

From #3624 (comment):

we've come to the conclusion that [...] support for processors is in conflict with proposed performance improvements such as described in #3565. Let's not document processors, as we would like to replace them with more APM-specific functionality/config.

In addition to not documenting them, we should explicitly deprecate their use and (when possible) log deprecation notices (#4284).