Remove obsolete icons-related CSS

These rules aren't used since commit c5c9efee1.

Note that there's still an `icon-budget` element in the
`_investment_show` partial. The rules for this icon are already defined
inside the rules for the `.budget-investment-show` selector.

The `icon-budget` and `icon-proposals` HTML classes are still used in
the "Following" tab (the code uses the `followable_icon` method), but in
this case the `.budget-investment` or `.proposal` selectors aren't
present, so the properties weren't applied here either.

Similarly, there are elements with the `icon-debates` and
`icon-proposals` HTML classes in the dashboard area, but they aren't
inside elements matching the `.debate` or `.proposal` selectors either.

And there's an element with the `icon-debate` HTML class in the
legislation area. Once again, it isn't inside an element matching the
`.debate` selector.

So can safely remove this code.

app/assets/stylesheets/participation.scss

@@ -512,28 +512,6 @@
     }
   }
 
-  .icon-debates,
-  .icon-proposals,
-  .icon-budget {
-    font-size: rem-calc(18);
-    line-height: $line-height;
-    margin-left: rem-calc(6);
-    top: 0;
-  }
-
-  .icon-debates {
-    color: $debates;
-  }
-
-  .icon-proposals {
-    color: $proposals-dark;
-  }
-
-  .icon-budget {
-    color: $budget;
-    font-size: $small-font-size;
-  }
-
   .debate-info,
   .proposal-info,
   .investment-project-info,