Remove redundant code / clean codebase

[timbocode]

Although much of the code from the parts of Rank Math not carried over to Classic SEO have already been removed, it is known that some redundant code remains, particularly in JS and CSS.

The opportunity should be taken to remove any code that is specific to PHP 5.6 as Classic SEO does not support any version of PHP below 7.0.

Similarly, the entire codebase needs to be examined to see where improvements and efficiencies can be made.

Massive amount of CPSEO settings appear in Post's Custom Metadata is potentially an area where such improvements can be made. Although PR #76 partially addresses this issue, it only hides the meta data but does not stop it from being created. Ways of reducing the amount of data will be examined as part of issue 41.