refactor: Drop support for using propTypes to define observedAttrs

[rschristian]

No one's using propTypes today really, and whilst this costs very little, I don't think it's serving anyone any value.

[github-actions]

Size Change: -31 B (-0.79%)

Total Size: 3.92 kB

Filename	Size	Change
dist/preact-custom-element.js	1.28 kB	-11 B (-0.85%)
dist/preact-custom-element.mjs	1.29 kB	-10 B (-0.77%)
dist/preact-custom-element.umd.js	1.35 kB	-10 B (-0.74%)

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