remove "`beam-migrate` tool" from docs

[peterbecich]

Following #684

it is still here https://haskell-beam.github.io/beam/schema-guide/tool/

The markdown file has already been deleted: https://github.com/haskell-beam/beam/edit/master/docs/schema-guide/tool.md
Does something need to be regenerated?

[LaurentRDC]

Something does need to be regenerated. This is not currently automated using GitHub actions, but it should

Edit: created a task for myself: #727