gtk3: Don't hardcode mate-panel tray icon size

Fixes: #104

Author: jnsh

common/gtk-3.0/3.20/sass/_applications.scss

@@ -693,7 +693,6 @@ panel-toplevel.background {
 
   na-tray-applet {
     -NaTrayApplet-icon-padding: 0;
-    -NaTrayApplet-icon-size: 16px;
   }
 }
 

common/gtk-3.0/3.22/sass/_applications.scss

@@ -750,7 +750,6 @@ panel-toplevel.background {
 
   na-tray-applet {
     -NaTrayApplet-icon-padding: 0;
-    -NaTrayApplet-icon-size: 16px;
   }
 }
 

common/gtk-3.0/3.24/sass/_applications.scss

@@ -747,7 +747,6 @@ panel-toplevel.background {
 
   na-tray-applet {
     -NaTrayApplet-icon-padding: 0;
-    -NaTrayApplet-icon-size: 16px;
   }
 }