Backend: Remove `readme_html` field from CI Catalog ResourceType

The following discussion from !139983 (merged) should be addressed:

• @lma-git started a discussion:

  Currently the FE is using the readmeHtml field from ResourceType, which returns the README content from the project's default branch/HEAD commit. This won't be useful after the FE updates their queries to get the versioned README from VersionType.readmeHtml. After FE completes #436401 (closed), we can remove this field from ResourceType.

Proposal

Remove readme_html field from Types::Ci::Catalog::ResourceType.

Implementation Table

Group	Issue Link
backend	Backend: Make ciCatalogResource accept a fullpa... (#429100 - closed)
backend	Backend: Support fetching versioned README for ... (#429279 - closed)
frontend	Frontend: Support fetching versioned README for... (#436401 - closed)
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