Remove WP_Cache non_existant_objects array

[duck_]

This was introduced back in [3021] "for options, which has its own notoptions cache these days" (ryan), to help reduce file system hits.

Right now it appears to just take up a bit of extra memory and cause incorrect figures on cache misses (assuming that we do want to report a 2nd request for an uncached item as a miss).

Noticed this as a result of investigation prior to #15812, I saw that stats was showing no increase in cache misses and I thought this a little strange but didn't question it too much.

[duck_]

Initial patch by inspection, needs testing and feedback in case I missed a purpose.

[ryan]

Looks good.

[duck_]

In [18682]:

Remove the non_existent_objects property from WP_Object_Cache. Fixes #16274.