Remove the azure_store extension directory, its Azure SDK CMake wiring, related Evergreen CI tasks, and Python test suite references. This eliminates the Azure SDK for C++ dependency and Azure-specific CI infrastructure.

Tasks
1. Delete ext/storage_sources/azure_store/ directory
2. Delete cmake/third_party/azure_sdk.cmake
3. Remove ENABLE_AZURE config option from cmake/configs/base.cmake
4. Remove conditional Azure SDK include from CMakeLists.txt
5. Remove azure_store subdirectory reference from ext/CMakeLists.txt
6. Remove configure_flags_tiered_storage_azure anchor and its usages from test/evergreen.yml
7. Remove Azure/GCP combined tasks: azure-gcp-tiered-storage-extensions-test, azure-gcp-tiered-catch2-unittest-test, azure-gcp-tiered-test-small
8. Remove azure-gcp task references from buildvariants section in test/evergreen.yml
9. Remove Azure-specific test scenarios from test/suite/helper_tiered.py

Definition of Done
- ext/storage_sources/azure_store/ directory is deleted
- cmake/third_party/azure_sdk.cmake is deleted
- ENABLE_AZURE CMake option is removed from cmake/configs/base.cmake
- Azure SDK conditional include is removed from root CMakeLists.txt
- Azure subdirectory reference is removed from ext/CMakeLists.txt
- All Azure-specific Evergreen tasks (combined with GCP) and their build variant references are removed
- All azure_store references are removed from Python test suite files
- cmake --build build succeeds (without -DENABLE_AZURE=1)
- dist/s_all validation passes