[SPARK-55800][PYTHON] Remove the unused type check for datetime.date

### What changes were proposed in this pull request?

We check if an object is an instance of `datetime.date` when it could be `datetime.datetime` - that's unnecessary and pointless because `datetime.datetime` is a subclass of `datetime.date`. It's documented in python docs so it's guaranteed.

We can also safely convert a `datetime.datetime` object directly. (We have been doing that for a long time).

### Why are the changes needed?

Remove unnecessary and misleading logic.

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

CI.

### Was this patch authored or co-authored using generative AI tooling?

No.

Closes apache#54581 from gaogaotiantian/fix-expression-datetime.

Authored-by: Tian Gao <gaogaotiantian@hotmail.com>
Signed-off-by: Ruifeng Zheng <ruifengz@apache.org>

Author: gaogaotiantian

python/pyspark/sql/connect/expressions.py

@@ -298,11 +298,8 @@ def __init__(self, value: Any, dataType: DataType) -> None:
                 assert isinstance(value, (str, np.str_))
                 value = str(value)
             elif isinstance(dataType, DateType):
-                assert isinstance(value, (datetime.date, datetime.datetime))
-                if isinstance(value, datetime.date):
-                    value = DateType().toInternal(value)
-                else:
-                    value = DateType().toInternal(value.date())
+                assert isinstance(value, datetime.date)
+                value = DateType().toInternal(value)
             elif isinstance(dataType, TimeType):
                 assert isinstance(value, datetime.time)
                 value = TimeType().toInternal(value)