Remove contain property (#31259)

This was telling the browser that the children of this div will never go
out of the bounding rect of the div. This is incorrect because the
expand button is supposed to sit at the edge of the space panel with
half of it outside the bounding rect.

res/css/structures/_LeftPanel.pcss

@@ -18,9 +18,6 @@ Please see LICENSE files in the repository root for full details.
     flex-direction: column;
     max-width: 50%;
     position: relative;
-
-    /* Contain the amount of layers rendered by constraining what actually needs re-layering via css */
-    contain: layout paint;
 }
 
 .mx_LeftPanel_wrapper,