Remove outdated storage-types-implementation-plan.md

The implementation plan was heavily outdated with:
- Old type names (<content>, <xblob>, <xattach> vs <hash@>, <blob@>, <attach@>)
- Wrong hash algorithm (SHA256 vs MD5)
- Wrong paths (_content/ vs _hash/)
- References to removed HashRegistry table

All relevant design information is now in storage-types-spec.md.
Implementation details (ObjectRef API, staged_insert) will be documented
in user-facing API docs when implemented.

Co-authored-by: dimitri-yatsenko <dimitri@datajoint.com>

Author: claude