Noissue - Do not keep list of unconditional selectors

_getUnconditionalSelectors() returns the value of _unconditionalSelectors after generating it if necessary from the keys of _filterBySelector. There are two places in which _getUnconditionalSelectors() is called: in _getDefaultStyleSheet(), at the time of the generation of the default style sheet, and then in getStyleSheet() itself, when either the includeSelectors option or the includeExceptions option is set. In the former case, the default style sheet is cached once generated; therefore, _unconditionalSelectors is never needed again. The latter happens only when element hiding debugging is turned on (#139 (closed)) or when DevTools is open so the browser extension can print the exceptions (issue 6428); even in this case, it should be quick to obtain the value on the fly by iterating over the keys of _filterBySelector.

Since we are now trying to tackle the memory usage for eyeo/adblockplus/libadblockplus>, it should make sense to lose _unconditionalSelectors.