Revert [25202] and enforce that wp_add_inline_style() does not want <style> tags.

Prior to 3.7, these tags were not printed (and thus needed to be provided), but only in the admin and when concatenation was enabled. They should never be required. Strip them when we find them and issue a notice for incorrect usage.

props atimmer, georgestephanis.
fixes #24813.

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