perf(): remove shims for legacy browsers no longer supported in v6

[liamdebeasi]

Pull request checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• Build (npm run build) was run locally and any changes were pushed
• Lint (npm run lint) has passed locally and any fixes were made for failures

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

Framework v5 enabled support for certain shims for legacy browsers, but these legacy browsers are no longer supported in Framework v6 so the shims are not needed.

What is the new behavior?

• Undoes part of chore(build): set stencil extras config #21877
• Removed shims for css variables, safari 10, and shadow dom
• Apps should get a very minor perf bump since the JS checks for whether or not to load the shims are no longer present.

Note: dynamicImportShim is still required because we support Firefox 63+ in v6, but dynamic imports were not added until Firefox 67.

Does this introduce a breaking change?

• [y] Yes
• No

Note: This is technically a breaking change, but we have already noted this change in https://github.com/ionic-team/ionic-framework/blob/next/BREAKING.md#browser-and-platform-support so as long as developers are running the supported browser and platform versions this change should be invisible to them.

Other information