Remove https://bugzil.la/504622 from Wall of Browser Bugs (#21260)

It's fixed in Firefox 53! Yay!
Refs #12359
[ci skip]

docs/_data/browser-bugs.yml

@@ -158,16 +158,6 @@
   origin: >
     Bootstrap#20161
 
--
-  browser: >
-    Firefox
-  summary: >
-    `min-width: 0` has no effect on `<fieldset>`
-  upstream_bug: >
-    Mozilla#504622
-  origin: >
-    Bootstrap#12359
-
 -
   browser: >
     Firefox