cleanup(misc): remove unused migrations (#28591)

This PR removes migrations that are unused in our packages.

Other changes:

- Update the `remove-migrations` generator to remove all files inside
the migration directory, rather than just the implementation, spec file,
and `__snapshots__` directory. There were some helpers that were not
removed due to this problem.
- Add a check to the `assertValidMigrationPaths` test util (used in
`migrations.spec.ts` test files) to ensure that all paths inside
`./src/migrations` are included in `migrations.json` file. There were a
few migrations missing in the `@nx/expo` package, and we never run them
nor cleaned them up.


**Note:** This has no effect on users and is purely a clean-up.

## Current Behavior
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## Expected Behavior
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-->

## Related Issue(s)
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merged. -->

Fixes #

packages/angular/.eslintrc.json

@@ -59,6 +59,7 @@
               "prettier",
               "typescript",
               // These are installed by generators
+              "@schematics/angular",
               "@angular-devkit/core",
               "@angular-devkit/architect",
               "@angular-devkit/build-angular",