[MNT] Tidy up utils (#2093)

* remove Theta

* remove Panel

* revert split_series

* remove test

aeon/utils/__init__.py

@@ -1,9 +1,6 @@
 """Utility functionality."""
 
 __all__ = [
-    "get_cutoff",
-    "update_data",
-    "get_window",
     "split_series",
     "ALL_TIME_SERIES_TYPES",
     "COLLECTIONS_DATA_TYPES",
@@ -21,4 +18,3 @@
 )
 from aeon.utils._show_versions import show_versions
 from aeon.utils._split import split_series
-from aeon.utils.index_functions import get_cutoff, get_window, update_data

aeon/utils/conversion/_convert_collection.py

@@ -14,7 +14,6 @@
 For the seven supported, this gives 42 different converters.
 Rather than using them directly, we recommend using the conversion function
 convert_collection.
-Legacy code supported "dask_panel" but it is not actually used anywhere; thus, removed.
 """
 
 from collections.abc import Sequence