Remove the s3_store extension directory, its AWS SDK CMake wiring, related Evergreen CI tasks, and Python test suite references. This eliminates the AWS SDK for C++ dependency and S3-specific CI infrastructure.

Tasks
1. Delete ext/storage_sources/s3_store/ directory
2. Delete cmake/third_party/aws_sdk.cmake
3. Remove ENABLE_S3 config option from cmake/configs/base.cmake
4. Remove aws_sdk include from CMakeLists.txt
5. Remove S3 subdirectory reference from ext/CMakeLists.txt
6. Remove configure_flags_tiered_storage_s3 anchor and usages from test/evergreen.yml
7. Remove S3-specific Evergreen tasks (unit-test-hook-tiered-s3, s3-tiered-test-small, s3-tiered-catch2-unittest-test, csuite-timestamp-abort-test-s3, bench-tiered-push-pull-s3)
8. Remove S3 task references from build variants section in test/evergreen.yml
9. Remove S3-specific test scenarios from test/suite/helper_tiered.py
10. Remove S3-specific conditional logic from test/suite/test_tiered*.py files

Definition of Done
- ext/storage_sources/s3_store/ directory is deleted
- cmake/third_party/aws_sdk.cmake is deleted
- ENABLE_S3 CMake option is removed from cmake/configs/base.cmake
- AWS SDK include is removed from root CMakeLists.txt
- S3 subdirectory reference is removed from ext/CMakeLists.txt
- All S3-specific Evergreen tasks and their build variant references are removed
- All s3_store references are removed from Python test suite files
- cmake --build build succeeds (without -DENABLE_S3=1)
- dist/s_all validation passes