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