chore: drop commitlint usage

[trentm]

This removes usage of commitlint (via 'npm run lint:commit') to strictly
lint commit messages. The spirit was considered good, but too often was
a tripping hazard. The commit guidelines have been edited to attempt to
encourage focus the substance of good commit messages.

Fixes: #1887

[apmmachine]

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Build stats

• Build Cause: Pull request #1901 updated

• Start Time: 2020-12-01T20:08:14.614+0000

• Duration: 21 min 31 sec

Test stats 🧪

Test	Results
Failed	0
Passed	16398
Skipped	0
Total	16398

[astorm]

The PR itself looks solid, so approving.

Also, talking out loud, I noticed that that the CI check apm-ci/linting/pr-merge is still running/failing, so we'll want to figure what we need to do to get that off the list of checks made prior to a PR.

[astorm]

@trentm Question -- I just realized some of our jenkins tests invoke the commit lint -- specifically the test_basic.sh script. We'll probably want to remove this invocation as well https://github.com/elastic/apm-agent-nodejs/blob/master/.ci/scripts/test_basic.sh#L11

Realized there was an invocation we'll need to update.