Drop jmxmp support from jmx-metrics and jmx-scraper

[trask]

JMXMP appears to be a dead protocol, and the dependency are relying on to support it isn't in maven central and hasn't been updated in 6 years old.

[SylvainJuge]

I agree, we should drop it, I've opened #2502 to remove it:

• it was added in jmx-scraper just because it was there in jmx-metrics
• it was never explicitly tested
• it was using an "obscure" terracotta maintained version, and there isn't any "official version" published in maven central
• we had no request/feedback that indicates someone is using it
• if needed, there is a simple workaround: adding the missing extra jar when running jmx-scraper/jmx-metrics. This is exactly what is being done for JBoss that provides an JMX over HTTP client library that is part of the Jboss server distribution and depends on the version, and thus that we can't simply include it.