Remove `host.user.*` fields

[ebeahan]

Summary

Due to minimal use and the new approach for users detailed in RFC 0007, host.user.* will be removed in the next ECS major version.

Background

A decision was made in RFC 0007 to deprecate the host.user* fields.

This has been noted in the ECS docs,

Tasks

• Add host.user.* deprecation notice in field reuse description #1422 Include deprecation notice on the description for host.user.* once Allow custom descriptions for self-nesting reuses #1174 has been implemented.
• Remove deprecated host.user.* fields #1439 Remove host.user.* in 8.0.0