Bug 1751484 - Remove expensive, barely perceptible transition that triggers at unwanted times. r=dao

See https://bugzilla.mozilla.org/show_bug.cgi?id=1751484#c3, this
transition triggers on all tabs whenever the visibility of an ancestor
changes (like it does for DOM full-screen).

I think this is barely perceptible (it only applies visibility: hidden
for one frame, tab open animation looks equally neat on my machine
without it), so removing this is easier, but let me know if you instead
want to add an attribute to the tab to stop applying the transition, or
something of that sort around here:

  https://searchfox.org/mozilla-central/rev/f7eeca4e34daea900fbedbc392c8c99f90923143/browser/base/content/tabbrowser-tabs.js#171

Differential Revision: https://phabricator.services.mozilla.com/D136667

browser/base/content/browser.css

@@ -202,14 +202,6 @@ panelview[mainview] > .panel-header {
   visibility: hidden;
 }
 
-.tab-icon-image[fadein],
-.tab-close-button[fadein],
-.tabbrowser-tab[fadein]::after,
-.tab-background[fadein] {
-  /* This transition is only wanted for opening tabs. */
-  transition: visibility 0ms 25ms;
-}
-
 .tab-icon-pending:not([fadein]),
 .tab-icon-image:not([fadein]),
 .tab-close-button:not([fadein]),