Remove deprecated "page.header.cta_url" setting

[iBug]

Found this lying around while doing bde92b6. Thought I'd just remove it but hesitated for these reasons:

• I'm a fan of Semantic Versioning and this is a breaking change for existing users.
  • Material for MkDocs is another theme that I use extensively, and it's doing a great job of following SemVer.
• Too minor a change for bumping the "major" version (i.e. v5.0.0).
  • As far as I understand, a website theme is intended as a "root dependency" (facing end users directly), so there was no plan to follow SemVer from the beginning. The "major" version of this project is thus intended as "major overhauls" or "reincarnations (not really)".

Context:

• Already deprecated since 4.13.0 (September 2018).

• Not many sites would be affected: I glanced through the search results and there's not many live sites using Gemfile with MM version specified as "~> 4.something" (i.e. audience of SemVer).

  Most repos are full forks so they're not affected, and those who use the remote theme method without specifying a version will be affected nevertheless (regardless of whether we bump the version number).

Given the considerations above, I'm inclined to push this forward. Though for the first time of doing this, I'd like to ask for your input, @mmistakes.

[mmistakes]

@iBug Your logic above makes sense to me. I loosely followed SemVer with each release.

• MAJOR releases were for redesigns or rewrites… didn’t happen often.
• MINOR enhancements or breaking changes
• PATCHES documentation changes or minor bug fixes