Cleanup HstCommmentProcessor service code

The code in the HST comments processor should be cleaned up a bit:

• Use normal class properties for 'log' and 'HST' constants
• Remove the fallback to DOM walking, which was only needed for IE. All supported browsers (Chrome, Firefox, Edge and Safari) support XPath parsing via 'document.evaluate' (see https://developer.mozilla.org/en-US/docs/Web/API/Document/evaluate#Browser_compatibility)