chore: delete unused MDX migration script (#26)

This script was a one-off utility for migrating MDX files from Nextra to Rspress. It is no longer needed and is not part of the build pipeline.

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>
Co-authored-by: sunnylqm <615282+sunnylqm@users.noreply.github.com>

Author: sunnylqm