Code clean-up: class-methods-use-this

[lindapaiste]

Fixes some violations of eslint rule class-methods-use-this which were suppressed.

• Use the isAbsoluteUrl and getModelPath utilities from modelLoader instead of duplicating them in multiple classes.
• Removed one eslint-disable which was not actually needed.
• Use boolean || instead of class method checkUndefined to set defaults in FaceApi. I wanted to use the nullish coalescing ?? but the eslint version is too outdated to support it!

---

The FaceApi code could be cleaned up further with object spreading, but the behavior would differ slightly if the user explicitly provides a falsey value. It could be reduced to:

this.config = {
  ...DEFAULTS,
  ...options,
  MODEL_URLS: {
    ...DEFAULTS.MODEL_URLS,
    ...options.MODEL_URLS
  }
}

[joeyklee]

LGTM! 🏅 Thanks for digging in here.

[lindapaiste]

Let's hold off on merging this one because I really don't love the || in FaceApi. ?? would have been better, but I want to create a general utility for merging options with defaults.

As for the isAbsoluteUrl method in DCGAN, that is addressed in #1376