Rich Text: Remove overridden underline format

[ntsekouras]

Currently the underline format is overridden from core in order to add an option in block's toolbar formats dropdown.

In core this button (alongside with the justify one) were removed years ago. TLDR;

Underline - It introduces a potentially bad user experience issue: readers confusing the underline text with a link. Underlined text is broadly interpreted as web links.

Additionally, right now is buggy, because it doesn't handle the shortcuts for it (Command + U).

I would suggest to stop overriding the format and align with core.

Steps to reproduce

1. Open the post editor and insert some text in a paragraph
2. Open the formats dropdown in the block's toolbar
3. Observe the Underline format.