style(coffee): remove superfluous parentheses

the following reasons for this PR are in priority order:

1) consistency, the following files already omit the unneeded parens
     /templates/coffeescript/service/provider.coffee
     /templates/coffeescript-min/service/provider.coffee
     /templates/coffeescript-min/directive.coffee

2) this change is similar to unneeded semicolons (e38124ee)

3) all function examples that don't take arguments on
   coffeescript.org also don't have empty parens

My team and I use this generator daily, and are trying to adhere to
the following: https://github.com/polarmobile/coffeescript-style-guide