Remove deprecated shell_streams shim

shell_streams (plural) was deprecated in ipykernel 6.0 (2021) in favor of
shell_stream (singular). Remove the trait, default handler, observer, and
associated test.

Author: erikgaas

ipykernel/kernelbase.py

@@ -110,45 +110,10 @@ def _update_eventloop(self, change):
     profile_dir = Instance("IPython.core.profiledir.ProfileDir", allow_none=True)
     shell_stream = Instance(ZMQStream, allow_none=True)
 
-    shell_streams: List[t.Any] = List(
-        help="""Deprecated shell_streams alias. Use shell_stream
-
-        .. versionchanged:: 6.0
-            shell_streams is deprecated. Use shell_stream.
-        """
-    )
-
     implementation: str
     implementation_version: str
     banner: str
 
-    @default("shell_streams")
-    def _shell_streams_default(self):  # pragma: no cover
-        warnings.warn(
-            "Kernel.shell_streams is deprecated in ipykernel 6.0. Use Kernel.shell_stream",
-            DeprecationWarning,
-            stacklevel=2,
-        )
-        if self.shell_stream is not None:
-            return [self.shell_stream]
-        return []
-
-    @observe("shell_streams")
-    def _shell_streams_changed(self, change):  # pragma: no cover
-        warnings.warn(
-            "Kernel.shell_streams is deprecated in ipykernel 6.0. Use Kernel.shell_stream",
-            DeprecationWarning,
-            stacklevel=2,
-        )
-        if len(change.new) > 1:
-            warnings.warn(
-                "Kernel only supports one shell stream. Additional streams will be ignored.",
-                RuntimeWarning,
-                stacklevel=2,
-            )
-        if change.new:
-            self.shell_stream = change.new[0]
-
     control_stream = Instance(ZMQStream, allow_none=True)
 
     debug_shell_socket = Any()

tests/test_kernel_direct.py

@@ -103,18 +103,6 @@ async def test_direct_debug_request(kernel):
     assert reply["header"]["msg_type"] == "debug_reply"
 
 
-async def test_deprecated_features(kernel):
-    with warnings.catch_warnings():
-        warnings.simplefilter("ignore", DeprecationWarning)
-        header = kernel._parent_header
-        assert isinstance(header, dict)
-        shell_streams = kernel.shell_streams
-        assert len(shell_streams) == 1
-        assert shell_streams[0] == kernel.shell_stream
-        warnings.simplefilter("ignore", RuntimeWarning)
-        kernel.shell_streams = [kernel.shell_stream, kernel.shell_stream]
-
-
 async def test_process_control(kernel):
     from jupyter_client.session import DELIM