This commit removes the method Posts::getTerms() as it existed to add a
compatibility layer for sites running WP < 4.5.0 and we don't support
this version anymore. The signature of the WP get_terms() function was
changed in version 4.5.0 and that is why this compatibility layer was
needed.
The integration test class for this method was also removed. The only
place where this method was used, AutomatedLatestContent::getTerms(),
now call WPFunctions->getTerms() directly. A very basic integration test
was added to cover the happy path of AutomatedLatestContent::getTerms().
[MAILPOET-4225]
