Remove adb cleanup from snippet and JSON-RPC client finalizers

[xpconanfan]

Problem

ClientBase.__del__ and JsonRpcClientBase.__del__ run close_connection() / disconnect(), which issue adb commands (adb forward --list, adb forward --remove). Doing blocking subprocess work from a finalizer is unreliable:

• It runs whenever the garbage collector decides, on whichever thread triggered the collection (thread-pool workers, the logcat processor thread), and at interpreter shutdown — the source of the Exception ignored in: ClientBase.__del__ noise.
• After a failed stop(), a stale event client's finalizer can remove the port forward of a new client that reused the same host port.
• In the unit test suite, leaked clients fire real adb processes long after the tests' adb mocks are gone. Where those land depends on allocation patterns, so an unrelated test-file removal (Remove the deprecated callback_handler #1037) was enough to hang the Windows + Python 3.14 CI job.

Fix

Delete the two __del__ methods. Explicit cleanup is already the supported path: AndroidDevice teardown → services.stop_all() stops every snippet client, and unload_snippet / restore_server_connection go through stop() / close_connection(). The socket itself is closed by Python when the object is collected; the finalizers only ever added the adb calls.

Tests

• The one test that invoked __del__ directly now calls close_connection().
• Dropped the double-side_effect workaround in jsonrpc_client_base_test.test_disconnect_raises that only existed to survive the finalizer.
• Real adb launches during a full pytest tests run on master (measured with a temporary subprocess.Popen tracer): 18 → 0, 3/3 runs.

Notes

• Independent of Remove the deprecated callback_handler #1037; the host_port = None test cleanup added there stays in place (removing it before this lands would re-trigger the CI hang).
• Release note: snippet / JSON-RPC clients no longer attempt adb cleanup on garbage collection. Call stop() / close_connection(), which AndroidDevice teardown already does.