chore: remove orphaned learning-objectives snippet

[memosr]

Summary

Removes docs/snippets/learning-objectives.mdx — a 253-line snippet containing 46 broken internal links to a /learn/ directory that no longer exists in this repo.

Background

In PR #1042 (Feb 2026), the Learn section was removed from this repo and migrated to a separate repo at github.com/base/learn-docs. All 180 /learn/ files were deleted at that time, but the learning-objectives.mdx snippet that referenced them was left behind.

Why delete instead of fix?

I investigated all three options:

Option	Outcome
Fix each link to point to current docs	Not viable — 0 of 46 topics have direct equivalents. The snippet covers a Solidity curriculum (Hardhat, ERC-20/721, Solidity basics) that doesn't exist in this repo's current scope
Point links to base/learn-docs archive	Would preserve dead code with no consumer
Delete the orphaned snippet	Clean removal — recommended

Verification

• ✅ Repo-wide grep for learning-objectives returns zero references in any .mdx, .md, .json, .ts, or .tsx file
• ✅ The snippet has no imports anywhere
• ✅ Git history shows it was brought in with the initial Mintlify migration (June 2025) and never touched since
• ✅ All 46 linked topics confirmed missing in current docs/ tree

Impact

• No rendered pages affected (no consumers)
• Reduces 46 broken internal links surface area
• Removes dead code

Happy to convert to a redirect/external link approach if maintainers prefer — just let me know.

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/2	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 1 Sum 2