[flutter_migrate] Remove source (#9847)

The package has been discontinued on pub.dev.

Context for removal:
- It was never fully launched (e.g., the version on pub didn't follow the standard release process).
- It has not been actively developed in more than a year, and has been subject to bitrot ([example](flutter/flutter#121257)).
- There are currently no plans to re-staff the project in the foreseeable future in order to get it to a stable state.

Since it's a use-as-needed tool rather than something that applications actively depend on, and it has <100 downloads per month, we are not doing a deprecation period.

Author: stuartmorgan-g

.github/labeler.yml

@@ -38,11 +38,6 @@
     - any-glob-to-any-file:
       - packages/flutter_lints/**/*
 
-'p: flutter_migrate':
-  - changed-files:
-    - any-glob-to-any-file:
-      - packages/flutter_migrate/**/*
-
 'p: flutter_plugin_android_lifecycle':
   - changed-files:
     - any-glob-to-any-file:

CODEOWNERS

@@ -10,7 +10,6 @@ packages/cross_file/**                                   @ditman
 packages/extension_google_sign_in_as_googleapis_auth/**  @ditman
 packages/file_selector/**                                @stuartmorgan-g
 packages/flutter_lints/**                                @chunhtai
-packages/flutter_migrate/**                              @stuartmorgan-g
 packages/flutter_template_images/**                      @stuartmorgan-g
 packages/go_router/**                                    @chunhtai
 packages/go_router_builder/**                            @chunhtai