[ci] Move implicit fem android demo to master branch (taichi-dev#6463)

If we pin on a commit taichi-aot-demo it's hard to catch all the errors.

Let's see if things already exploded... 

Issue: #

### Brief Summary

.github/workflows/scripts/aot-demo.sh

@@ -16,14 +16,9 @@ function build-and-smoke-test-android-aot-demo {
     export TAICHI_REPO_DIR=$(pwd)/taichi
 
     rm -rf taichi-aot-demo
+    # IF YOU PIN THIS TO A COMMIT/BRANCH, YOU'RE RESPONSIBLE TO REVERT IT BACK TO MASTER ONCE MERGED.
     git clone https://github.com/taichi-dev/taichi-aot-demo
 
-    # Normally we checkout the master's commit Id: https://github.com/taichi-dev/taichi-aot-demo/commit/master
-    # As for why we need this explicit commit Id here, refer to: https://docs.taichi-lang.org/docs/master/contributor_guide#handle-special-ci-failures
-    pushd taichi-aot-demo
-    git checkout 6b8d22f2c38318cf7a7333dc17cff4ae7ee5e607
-    popd
-
     APP_ROOT=taichi-aot-demo/implicit_fem
     ANDROID_APP_ROOT=$APP_ROOT/android
     JNI_PATH=$ANDROID_APP_ROOT/app/src/main/jniLibs/arm64-v8a/