Remove x1.5 mobile icon size
This is necessary in order to ensure that developers can expect a consistent icon size when they develop for mobile and desktop, and so that they do not need to use workarounds (I currently don't use iconSizes presets at all because of this issue).
If applications need different sizings for mobile, it should be implemented explicitly.
This MR also adjusts sidebar item and floating action buttons to still remain the same size before the change.
This replaces !312 (closed) as well.