Remove intrinsic min-width of fieldset

At least firefox [1] and chrome [2] have a legacy/ intrinsic min-width
for the `fieldset` element. 
This removes it. The fix is especially useful on mobile.


[1] https://bugzilla.mozilla.org/show_bug.cgi?id=504622
[2] http://stackoverflow.com/questions/8084343/google-chrome-fieldset-overflow-bug