chore: remove Plans/ — implementation work complete

The Plans/ directory held the design / publishing plans we used during
development (customxml-implementation-plan.md, review-the-guide-at-swift-kahn.md).
The work is shipped: customXml landed in v1.2.0 on PyPI; both PyPI plans
are now historical record. Removing the directory in line with project
hygiene.

Author: Matthew Horoszowski