Remove stale comment about StaticRegister

Commit d470bd2cc (Make numeric registers setable, 2017-02-14) removed
the user-provided StaticRegister::operator= in favor of a set()
member function, so this comment is no longer valid.

src/register_manager.hh

@@ -34,7 +34,6 @@ protected:
 };
 
 // static value register, which can be modified
-// using operator=, so should be user modifiable
 class StaticRegister : public Register
 {
 public: