Cleanup a few repo docs (#1987)

The `MAINTENANCE.md` file in particular hasn't been updated in 3 years.

CONTRIBUTING.md

@@ -71,7 +71,7 @@ Each app should:
   file used throughout the repo. This file include a base set of analyzer
   conventions and lints.
 * Have no analyzer errors or warnings.
-* Be formatted with `flutter format`.
+* Be formatted with `dart format`.
 * Include at least one working test in its `test` folder.
 * Be wired into the list of projects in the CI scripts for [stable](tool/flutter_ci_script_stable.sh), 
   [beta](tool/flutter_ci_script_beta.sh), and [master](tool/flutter_ci_script_master.sh), 

MAINTENANCE.md

@@ -1,22 +0,0 @@
-# Sample app maintenance log
-
-This table records the last time someone from the Flutter team came did a
-maintenance pass on a particular sample (read through the code, updated it to
-match any new language/SDK features, etc.).
-
-| Sample                    | GH username        | Date Updated   |
-| :------------------------ | -----------------: | -------------: |
-| animations                | theacodes          | 10/7/19        |
-| add_to_app                |                    |                |
-| flutter_maps_firestore    |                    |                |
-| infinite_list             | filiph             | 5/13/20        |
-| isolate_example           | johnpryan          | 11/21/19       |
-| jsonexample               | redbrogdon         | 1/3/20         |
-| place_tracker             | johnpryan          | 8/13/20        |
-| platform_channels         |                    |                |
-| platform_design           | johnpryan          | 10/7/19        |
-| platform_view_swift       | redbrogdon         | 10/7/19        |
-| provider_counter          | filiph             | 8/13/20        |
-| provider_shopper          | filiph             | 8/13/20        |
-| testing_app               |                    |                |
-| veggieseasons             | filiph             | 8/13/20        |

README.md

@@ -38,7 +38,7 @@ If you have a general question about one of these samples or how to adapt its
 techniques for one of your own apps, try one of these resources:
 
 * [The FlutterDev Google Group](https://groups.google.com/forum/#!forum/flutter-dev)
-* [The Flutter Gitter channel](https://gitter.im/flutter/flutter)
+* [The FlutterDev Discord](https://discord.gg/rflutterdev)
 * [StackOverflow](https://stackoverflow.com/questions/tagged/flutter)
 
 If you run into a bug in one of the samples, please file an issue in the