Remove optional arguments from else() and end*() commands

This adds a brief explanation of the optional condition in the `else()`
and removes all legacy usages of `else()` and `end*()` commands.

Help/command/else.rst

@@ -8,3 +8,6 @@ Starts the else portion of an if block.
   else([<condition>])
 
 See the :command:`if` command.
+
+The optional ``<condition>`` argument is supported for backward compatibility
+only and is not evaluated.