Remove duplicate covering index page

This page exists in two locations in the docs-server repo:

• https://github.com/couchbase/docs-server/blob/release/6.0/modules/indexes/pages/covering-indexes.adoc
• https://github.com/couchbase/docs-server/blob/release/6.0/modules/n1ql/pages/n1ql-language-reference/covering-indexes.adoc

The content is almost identical, except for links to images. (The images are also duplicated.)

The page in the n1ql module is included in the nav, and the other one isn't, so I assume the one in the indexes module is superfluous.

Delete the superfluous page and images.