feat: remove deprecation when is is called with component definition

[xanf]

See #1532 for discussion

This PR removes deprecation warnings when is is called against component definition:

// generates warning
expect(wrapper.is('div')).toBe(true);
// does not generate warning
expect(wrapper.is(SomeComponent)).toBe(true)

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

The PR fulfills these requirements:

• It's submitted to the dev branch.
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue-test-utils/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

[lmiller1990]

Thanks! sorry I did not see these PRs earlier.