Remove integration test setup code and script (#5322)

# Motivation

The integration tests using PocketIC was added for stable structure
migration. The tests themselves are already removed in
#4814, while the test setup and
the github workflow is still there.

# Changes

* Remove the test setup code
* Remove the github workflow
* Remove test script
* Remove related documentation

# Tests

N/A

# Todos

- [x] Add entry to changelog (if necessary).

.github/workflows/build.yml

@@ -435,21 +435,6 @@ jobs:
           scripts/sns/aggregator/test-fast
       - name: Stop replica
         run: dfx stop
-  integration:
-    name: "Integration tests"
-    needs: build
-    runs-on: ubuntu-20.04
-    steps:
-      - uses: actions/checkout@v4
-      - name: Get docker build outputs
-        uses: actions/download-artifact@v4
-        with:
-          name: out
-          path: out
-      - name: Install pocket-ic
-        run: curl -sSL https://github.com/dfinity/pocketic/releases/download/3.0.1/pocket-ic-x86_64-linux.gz | gunzip | install -m 755 /dev/stdin pocket-ic
-      - name: Run integration tests
-        run: ./scripts/test-integration
   assets:
     name: "Upload assets"
     needs: build
@@ -522,7 +507,7 @@ jobs:
           ) | tee -a $GITHUB_STEP_SUMMARY
           (( wasm_size <= max_size )) || { echo "The WASM is too large" ; exit 1 ; }
   build-pass:
-    needs: ["build", "test-playwright-e2e-shard-1-of-2", "test-playwright-e2e-shard-2-of-2", "test-rest", "network_independent_wasm", "aggregator_test", "assets", "test-upgrade-downgrade", "test-test-account-api", "test-upgrade-stable", "integration"]
+    needs: ["build", "test-playwright-e2e-shard-1-of-2", "test-playwright-e2e-shard-2-of-2", "test-rest", "network_independent_wasm", "aggregator_test", "assets", "test-upgrade-downgrade", "test-test-account-api", "test-upgrade-stable"]
     if: ${{ always() }}
     runs-on: ubuntu-20.04
     steps:

CHANGELOG-Nns-Dapp-unreleased.md

@@ -43,6 +43,8 @@ proposal is successful, the changes it released will be moved from this file to
 
 #### Removed
 
+* Integration test setup code and related script and documentation.
+
 #### Fixed
 
 #### Security

TESTING.md

@@ -177,9 +177,3 @@ Tests are found in `rs/backend/src/accounts_store/tests.rs`.
 ```bash
 $ cargo test --lib
 ```
-
-### Integration tests
-
-```bash
-$ ./scripts/test-integration
-```