Remove extend from issuable sidebar css

Remove an extend usage from issue sidebar. Also remove some unrelated code.

Slight UI change to the '+ X more' button on the sidebar Participants section, which is now a regular tertiary button instead of blue on hover:

before	after

When not hovered it looks the same as currently:

Didn't change for 'Assign yourself' as that looked a bit strange as a button and has some overriding util classes already.