Delete old redundant encryption MDX file (#21834)

Delete old security/encryption docs file

This file path (`security/encryption.mdx` updated a year ago) is conflicting
with another file path (`security/encrpytion/index.mdx` updated 2 weeks
ago). This is causing the Vercel build to fail since the GitHub runner
cannot distinguish between the file `encryption.mdx` and the path
`encryption/index.mdx`

This one specifically was chosen for deletion due to it last being
updated a year ago vs `encryption/idex.mdx` being updated 2 weeks ago