Revert "feat: feature flags rfc (#4925)"

This reverts commit db50ab0.

CONTRIBUTING.md

@@ -36,7 +36,6 @@ and let us know if it's not up-to-date (even better, submit a PR with your  corr
   - [Updating all Dependencies](#updating-all-dependencies)
   - [Running CLI integration tests](#running-cli-integration-tests)
   - [API Compatibility Checks](#api-compatibility-checks)
-  - [Feature Flags](#feature-flags)
 - [Troubleshooting](#troubleshooting)
 - [Debugging](#debugging)
   - [Connecting the VS Code Debugger](#connecting-the-vs-code-debugger)
@@ -515,49 +514,6 @@ this API we will not break anyone, because they weren't able to use it. The file
 `allowed-breaking-changes.txt` in the root of the repo is an exclusion file that
 can be used in these cases.
 
-### Feature Flags
-
-Sometimes we want to introduce new breaking behavior because we believe this is
-the correct default behavior for the CDK. The problem of course is that breaking
-changes are only allowed in major versions and those are rare.
-
-To address this need, we have a feature flags pattern/mechanism. It allows us to
-introduce new breaking behavior which is disabled by default (so existing
-projects will not be affected) but enabled automatically for new projects
-created through `cdk init`.
-
-The pattern is simple:
-
-1. Define a new const under
-   [cx-api/lib/features.ts](https://github.com/aws/aws-cdk/blob/master/packages/%40aws-cdk/cx-api/lib/features.ts)
-   with the name of the context key that **enables** this new feature (for
-   example, `ENABLE_STACK_NAME_DUPLICATES`). The context key should be in the
-   form `module.Type:feature` (e.g. `@aws-cdk/core:enableStackNameDuplicates`).
-2. Use `node.tryGetContext(cxapi.ENABLE_XXX)` to check if this feature is enabled
-   in your code. If it is not defined, revert to the legacy behavior.
-3. Add your feature flag to
-   [cx-api/lib/future.ts](https://github.com/aws/aws-cdk/blob/master/packages/%40aws-cdk/cx-api/lib/future.ts).
-   This map is inserted to generated `cdk.json` files for new projects created
-   through `cdk init`.
-4. In your PR title (which goes into CHANGELOG), add a `(under feature flag)` suffix. e.g:
-
-    ```
-    fix(core): impossible to use the same physical stack name for two stacks (under feature flag)
-    ```
-5. Under `BREAKING CHANGES` in your commit message describe this new behavior:
-
-    ```
-    BREAKING CHANGE: template file names for new projects created through "cdk init" 
-    will use the template artifact ID instead of the physical stack name to enable 
-    multiple stacks to use the same name. This is enabled through the flag 
-    `@aws-cdk/core:enableStackNameDuplicates` in newly generated `cdk.json` files.
-    ```
-
-In the [next major version of the
-CDK](https://github.com/aws/aws-cdk/issues/3398) we will either remove the
-legacy behavior or flip the logic for all these features and then
-reset the `FEATURE_FLAGS` map for the next cycle.
-
 ## Troubleshooting
 
 Most build issues can be solved by doing a full clean rebuild: