fake-menu-button: remove min-width of borderless button

[melquan]

Bug Report

SKIN Version: 14.1.0

Description

There's a min-width of 88px on the borderless fake-menu-button so when there's no text, the icon scoots to the left and there's extra space on the right. This is being inherited from the .btn class.

Workaround

Override it to min-width: 0 in our app in the meantime.

Screenshots

[ianmcburnie]

Let's make sure we have a story in Skin storybook for this too.