Line break remains in RichText after deleting second line

[ktmn]

Describe the bug
After adding multiline text to a <RichText> component the content will always receive a trailing <br/> even after the text has been updated to no longer be multiline.

To Reproduce
Steps to reproduce the behavior:

1. Add a core/button block.
2. Add content "Hello{enter}{enter}World" to produce a button with multiple lines of text. (note: Already the content is Hello<br/>world<br/>, one <br/> too many)
3. (optional) Save, refresh...
4. Delete the text "World" to return the button to a single line of text
5. (optional) Save, refresh...
6. Both front and back end, the button's content is Hello<br/> when it should just be Hello

This behavior also exists when the block attribute is saved in comments, not pulled from dom, leaving this in the block comments: {"content":["Hello",{"type":"br","key":"_domReact1","ref":null,"props":{},"_owner":null,"_store":{}}]} when it could simply be {"content":["Hello"]}

Expected behavior

Don't leave a trailing <br/> when going multiline.

Also a <RichText> prop that prevents multiple lines altogether would be useful.

[designsimply]

Tested and confirmed that adding multiple lines of text to a button and then removing the extra lines leaves an extra <br />.

^{Seen at http://alittletestblog.com/wp-admin/post.php?post=13861&action=edit running WordPress 4.9.6 and Gutenberg 3.1.1 using Firefox 61.0 on macOS 10.13.5.}

[ellatrix]

Does this only happen in Firefox? Then this is a duplicate of #5872. Feel free to reopen if this in not the case.

[ktmn]

Nope, Chrome too.