Unnecessary 20px Macro Browser icons in images subfolder

"small_*.png" versions of the Macro Browser macro icons we created in images/icons/macrorowser/20px for the Insert Menu when the plan was to allow users to set the macros that appeared in the menu and have icons for each.

The macros can still be customized via web-resources but the icons can't, so we don't need these resources and if we end up needing them later we should create a sprite.