HTML API: Ensure attribute removals do not create self-closing tags

[sirreal]

Removing an attribute could accidentally change tag semantics. For example, removing attr from <svg><g /attr>ok produced <svg><g />ok, producing a self-closing flag on the g tag that was not present before.

Trac ticket: https://core.trac.wordpress.org/ticket/65372

Alternative to #12368.

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