Remove TCGPlayer deckbuilder integration

[ndepaola]

Description

The deckbuilder was deactivated by TCGPlayer on 2025-05-14:

It's a shame to see this go - thanks @claythearc for originally contributing this in #68 :)

Checklist

• I have installed pre-commit and installed the hooks with pre-commit install before creating any commits.
• I have updated any related tests for code I modified or added new tests where appropriate.
• I have manually tested my changes as follows:
  • Ensured TCGPlayer is no longer listed as a valid import site in the frontend
• I have updated any relevant documentation or created new documentation where appropriate.
  • None required

[ndepaola]

Presumably TCGPlayer doesn't let you create new decks (or modify decks?) anymore, but the existing deck library is still accessible at time of writing. However, our integration is broken at the moment - after investigating a bit further, it seems like this code still works if User-Agent is set.

I'll go ahead with bandaiding the User-Agent for now and will close this PR (probably reopening it when the TCGPlayer deck builder is fully dead).

[ndepaola]

she dead jim