[generator] Remove redundant mage targets in metricbeat generator magefile

[fearful-symmetry]

This is a first pass at pairing down the magefiles a bit. We already have a lot of stuff duplicated in target/, so this just removes anything we already have somewhere else. Most of the target names should be the same, however a few like GoTestIntegration have changed.

[fearful-symmetry]

jenkins, test this

[kaiyan-sheng]

hmm ci failed with Error: some files are not up-to-date. Run 'mage fmt update' then review and commit the changes. Modified: [heartbeat/docs/fields.asciidoc heartbeat/include/fields.go]

[fearful-symmetry]

What? Did I break something?

[fearful-symmetry]

jenkins, test this

[kvch]

Just curious, does the generated Beat rely on both make and mage?

[fearful-symmetry]

@kvch More or less everything depends simultaneously on make and mage. The two actually depend on each other. Make will all mage, mage will call make. Yah, it's ugly and I'd there's lots of things I'd like to refactor.