inlineStyles plugin should not remove id attribute from target element

[johnkenny54]

Removed code that deletes id attribute from elements.

Resolves #2065.

[strarsis]

Wouldn't an option be better? Subsequent plugins do not have the context to clean up the ID attribute, as it is removed by inline styles as being then unnecessary, as its associated styles were directly applied to the element.
PR that adds the options: #1165

[johnkenny54]

Having an option isn't safe and just adds complexity to the plugin. If the id is no longer referenced, it should be removed by cleanupIds. Currently cleanupIds disables itself in the presence of <style> elements, which is not ideal, but at least preferable to what many other plugins do, since it doesn't break anything. But there's no reason cleanupIds couldn't check the selectors to see which ids are referenced by styles.