compose: Fix misaligment of compose top right icons.

When composing a private message to a different recipient than the
current view, the go-to-compose-target icon looked vertically
misaligned with these icons. Fix this by removing the CSS rule that
made these other top-corner icons not centered within their row.

web/styles/compose.css

@@ -240,7 +240,6 @@
 #compose_top_right {
     display: flex;
     align-items: center;
-    margin-bottom: auto;
 
     & button {
         background: transparent;