Remove <style scoped>

This reverts 29cf39d and many follow-up
refinement commits due to lack of implementer interest. Most of the use
cases served by <style scoped> are better served by <style> elements
inside a shadow root (although the features do work differently, in
that shadow DOM enforces isolation from the cascade, whereas <style
scoped> does not).

Fixes #552.

Author: domenic