Prevent sticky type checking (E706) errors with older Vims (#681)

Vim used to throw the E706 error if you tried to change a variable's
type. This error can be triggered by vimwiki if, during the user
settings loop in `read_global_settings_from_user()`, the variable type
for `users_value` changes.

The vim error was removed in v7.4.1546 as part of
f6f32c38bf.

autoload/vimwiki/vars.vim

@@ -203,6 +203,8 @@ function! s:read_global_settings_from_user()
       call s:check_users_value(key, users_value, value_infos, 1)
 
       let g:vimwiki_global_vars[key] = users_value
+      " Remove users_value to prevent type mismatch (E706) errors in vim <7.4.1546
+      unlet users_value
     else
       let g:vimwiki_global_vars[key] = global_settings[key].default
     endif