Remove speculative context

[grarco]

Describe your changes

Closes #4074.

Removes the speculative shielded context and the shielded-sync cli command. The shielded-sync operations is now embedded directly in the cli commands that need it:

• Shielded transfer
• Unshielding transfer
• IBC transfer (when the source is shielded)
• Balance query (for shielded owners)
• Shielded rewards estimate

Checklist before merging

• [ ] If this PR has some consensus breaking changes, I added the corresponding breaking:: labels
  • This will require 2 reviewers to approve the changes
• [x] If this PR requires changes to the docs or specs, a corresponding PR is opened in the namada-docs repo
  • Relevant PR if applies: https://github.com/namada-net/namada-docs/pull/459
• [ ] If this PR affects services such as namada-indexer or namada-masp-indexer, a corresponding PR is opened in that repo
  • Relevant PR if applies: