revert changes to process default routes at build (#61241)
Reverts changes from #61173 & #60240 (while leaving the tests that were
added).
There are too many spots where considering `/default` routes as pages
needs to be carefully considered in different runtimes, and it turns out
that it's not actually needed to handle the case that it was originally
added for. I confirmed that the test that added the case it was intended
to fix (`parallel-routes-catchall-default`, along with the unit tests in
`normalize-catchall-routes`) are still passing as expected.
