Remove parens around equality checks

[syclik]

Summary:

There are redundant parens around some equality checks. See @bob-carpenter's comment

This example:

if ((eta == 1.0) &&
      stan::is_constant<typename stan::scalar_type<T_shape> >::value)

should be:

if (eta == 1.0 && is_constant<typename scalar_type<T_shape> >::value)

Current Version:

v2.16.0