Data-encoding: remove files that will be unused

What

This MR removes:

• data-encoding/.gitlab-ci.yml
• data-encoding/src/index.mld
• some dune and opam files

Why

.gitlab-ci.yml will never be used now that data-encoding is in tezos/tezos.

index.mld contained barely anything and broke the CI because apparently having multiple files with this name in the repository is an issue.

dune was empty and useless. dune-project was empty and will be replaced by the one at the root. dune-workspace will be replaced by the one at the root. .opam files will be generated by the manifest in another directory.

How

Using git rm, although actually one of the commits was cherry-picked from !12004 (merged).

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR