Remove text-shadow from Conpherence sticky header bar
Summary:
In dark mode, the current `text-shadow` of the header bar of Conpherence's Persistent Chat overlay offers a nice fuzzy feeling and is a great alternative to the consumption of beverages.
Remove it, similar to other text shadows in previous patches.
Refs: T15056
Test Plan:
* Go to http://phorge.localhost/settings/panel/display/ and set Accessibility to Dark Mode (Experimental)
* Install Conpherence via http://phorge.localhost/applications/view/PhabricatorConpherenceApplication/
* Create a Conpherence room
* In the notification section of the top bar, select the speech bubbles icon and enable "Persistent Chat", then look at the top bar of the chat window in the bottom right corner
Reviewers: O1 Blessed Committers, amybones, mainframe98
Reviewed By: O1 Blessed Committers, amybones, mainframe98
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T15056
Differential Revision: https://we.phorge.it/D26159