Stats: Use only defer for script tag

[kraftbj]

Fixes https://twitter.com/rick_viscomi/status/1331735748060524551

Changes proposed in this Pull Request:

• Drop obsolete use of type.
• Use only defer.
• Skip PHPCS validation since the script should be enqueued, but that's going to be the beginning of a deep rabbit hole given how old a lot of this code is. :)

Jetpack product discussion

Discussed with perfops in p1607034688079500-slack-C4GAQ900P

Does this pull request change what data or activity we track or use?

n/a

Testing instructions:

• Logged out on a site connected to Jetpack, ensure no JS errors and that the stats were counted.
• Should not be via AMP.

Proposed changelog entry for your changes:

• Stats: Update JavaScript loading to meet modern best practices.

[test-case-reminder]

Here are some suggested test cases for this PR.

Stats

• Visit home page, make sure that page view is recorded in stats
• Visit home page in AMP view
• Visit post page in non-AMP and AMP view

If you think that suggestions should be improved please edit the configuration file here. You can also modify/add test-suites to be used in the configuration file.

[jetpackbot]

	Warnings
⚠️	pre-commit hook was skipped for one or more commits

This is an automated check which relies on PULL_REQUEST_TEMPLATE. We encourage you to follow that template as it helps Jetpack maintainers do their job. If you think 'Testing instructions' or 'Proposed changelog entry' are not needed for your PR - please explain why you think so. Thanks for cooperation 🤖

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