cache: remove the largely obsolete general description

Examples of inconsistencies with the current state:

• references LRU of all entries, removed years ago
• references a non-existent lock (neglist)
• claims negative entries have a NULL target

It will be replaced with a more accurate and more informative
description.

In the meantime take it out so it stops misleading.

(cherry picked from commit 2f8a844635312b0f25028a87459fdd2d2a1cbfd6)