Remove EngineTimeSlot (#4608)

- It looks like this wrapper got created but never used.
- It does not provide much value over the proto values directly,
so I am just going to delete it.
- This also fixes the typing which incorrectly says that the
schedule is returned as a EngineTimeSlot, but it is actually a
QuantumTimeSlot.

While technically a breaking change, it would be quite surprising
if anyone is dependent on this wrapper class, as it has never been used
anywhere.

BREAKING CHANGE=Removed EngineTimeSlot without deprecation, (minimal to no usage).

cirq-google/cirq_google/__init__.py

@@ -71,7 +71,6 @@
     EngineJob,
     EngineProgram,
     EngineProcessor,
-    EngineTimeSlot,
     ProtoVersion,
     QuantumEngineSampler,
     get_engine,

cirq-google/cirq_google/engine/__init__.py

@@ -45,10 +45,6 @@
     EngineProcessor,
 )
 
-from cirq_google.engine.engine_timeslot import (
-    EngineTimeSlot,
-)
-
 from cirq_google.engine.engine_program import (
     EngineProgram,
 )

cirq-google/cirq_google/engine/engine_processor.py

@@ -22,7 +22,6 @@
 from cirq_google.api import v2
 from cirq_google.devices import serializable_device
 from cirq_google.engine import calibration
-from cirq_google.engine.engine_timeslot import EngineTimeSlot
 from cirq_google.serialization.serializable_gate_set import SerializableGateSet
 
 if TYPE_CHECKING:
@@ -277,7 +276,7 @@ def list_reservations(
         self,
         from_time: Union[None, datetime.datetime, datetime.timedelta] = datetime.timedelta(),
         to_time: Union[None, datetime.datetime, datetime.timedelta] = datetime.timedelta(weeks=2),
-    ) -> List[EngineTimeSlot]:
+    ) -> List[qenums.QuantumTimeSlot]:
         """Retrieves the reservations from a processor.
 
         Only reservations from this processor and project will be
@@ -307,7 +306,7 @@ def get_schedule(
         from_time: Union[None, datetime.datetime, datetime.timedelta] = datetime.timedelta(),
         to_time: Union[None, datetime.datetime, datetime.timedelta] = datetime.timedelta(weeks=2),
         time_slot_type: Optional[qenums.QuantumTimeSlot.TimeSlotType] = None,
-    ) -> List[EngineTimeSlot]:
+    ) -> List[qenums.QuantumTimeSlot]:
         """Retrieves the schedule for a processor.
 
         The schedule may be filtered by time.

cirq-google/cirq_google/json_test_data/spec.py

@@ -37,7 +37,6 @@
         'EngineJob',
         'EngineProcessor',
         'EngineProgram',
-        'EngineTimeSlot',
         'FSimPhaseCorrections',
         'NAMED_GATESETS',
         'ProtoVersion',