Delete long-obsolete tests and benchmarks (#7695)

unit-tests-ios benchmark-common-tasks-ios: used by the pre-cmake build system.

benchmark-row-accessor: never updated for core 6 and the thing it was
benchmarking isn't applicable any more.

benchmark-history-types: used to verify that moving the transaction logs into
the Realm file wouldn't cause performance problems. The code being benchmarked
no longer exists.

notifications-fuzzer: this was an important part of originally writing the
diffing logic for notifications, but everything it exposed was turned into
normal tests. It was never updated for core 6 and no bugs have ever been
reported in the code this was testing.

test_client_reset_query_based.cpp: query-based sync is no longer a thing.

test_optional.cpp: this was actually testing std::optional. There's still
some code in optional.hpp, but none of it was being tested here.

benchmark-transaction benchmark-prealloc benchmark-index benchmark-insert-add
performance: these were pre-Realm benchmarks against the old macro-based
TightDB API, which was deleted in 2016.

simple-connection: this was a manual testing tool from when sync first adopted
websockets.

protocol_compact: this was a tool for testing one version of the server against
a different version of the client, which we no longer care about.

test/CMakeLists.txt

@@ -68,7 +68,6 @@ set(CORE_TEST_SOURCES
     test_list.cpp
     test_mixed_null_assertions.cpp
     test_object_id.cpp
-    test_optional.cpp
     test_priority_queue.cpp
     test_replication.cpp
     test_safe_int_ops.cpp