Remove the gcp_store extension directory, its GCP SDK CMake wiring, remaining Evergreen CI infrastructure (GCP dependencies script), and Python test suite references. This eliminates the google-cloud-cpp dependency and GCP-specific CI infrastructure.

Tasks
1. Delete ext/storage_sources/gcp_store/ directory
2. Delete cmake/third_party/gcp_sdk.cmake
3. Remove ENABLE_GCP config option from cmake/configs/base.cmake
4. Remove gcp_sdk include from CMakeLists.txt
5. Remove gcp_store subdirectory reference from ext/CMakeLists.txt
6. Remove configure_flags_tiered_storage_gcp anchor from test/evergreen.yml
7. Remove install gcp dependencies function from test/evergreen.yml and its usage
8. Delete test/evergreen/install_gcp_dependencies.sh
9. Delete test/evergreen/gcp_auth.json
10. Remove GCP-specific test scenarios from test/suite/helper_tiered.py

Definition of Done
- ext/storage_sources/gcp_store/ directory is deleted
- cmake/third_party/gcp_sdk.cmake is deleted
- ENABLE_GCP CMake option is removed from cmake/configs/base.cmake
- GCP SDK include is removed from root CMakeLists.txt
- GCP subdirectory reference is removed from ext/CMakeLists.txt
- GCP dependencies installation function and related Evergreen scripts are removed
- All gcp_store references are removed from Python test suite files
- cmake --build build succeeds (without -DENABLE_GCP=1)
- dist/s_all validation passes