Remove RedisStorage from docstring

[c-bata]

Motivation

RedisStorage is removed at #4156

Description of the changes

Fix an error message while building the sphinx documentation.

reading sources... [100%] reference/generated/optuna.storages.RedisStorage
WARNING: autodoc: failed to import class 'RedisStorage' from module 'optuna.storages'; the following exception was raised:
Traceback (most recent call last):
  File "/usr/local/lib/python3.10/site-packages/sphinx/util/inspect.py", line 376, in safe_getattr
    return getattr(obj, name, *defargs)
AttributeError: module 'optuna.storages' has no attribute 'RedisStorage'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/usr/local/lib/python3.10/site-packages/sphinx/ext/autodoc/importer.py", line 98, in import_object
    obj = attrgetter(obj, mangled_name)
  File "/usr/local/lib/python3.10/site-packages/sphinx/ext/autodoc/__init__.py", line 306, in get_attr
    return autodoc_attrgetter(self.env.app, obj, name, *defargs)
  File "/usr/local/lib/python3.10/site-packages/sphinx/ext/autodoc/__init__.py", line 2804, in autodoc_attrgetter
    return safe_getattr(obj, name, *defargs)
  File "/usr/local/lib/python3.10/site-packages/sphinx/util/inspect.py", line 392, in safe_getattr
    raise AttributeError(name) from exc
AttributeError: RedisStorage

[cross32768]

LGTM!

[codecov-commenter]

Codecov Report

Merging #4232 (b337349) into master (3fcee2e) will increase coverage by 0.26%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4232      +/-   ##
==========================================
+ Coverage   89.67%   89.93%   +0.26%     
==========================================
  Files         162      169       +7     
  Lines       12705    13263     +558     
==========================================
+ Hits        11393    11928     +535     
- Misses       1312     1335      +23     

Impacted Files	Coverage Δ
optuna/storages/_heartbeat.py	91.11% <ø> (ø)
optuna/integration/botorch.py	96.52% <0.00%> (-0.87%)	⬇️
optuna/samplers/_random.py	100.00% <0.00%> (ø)
optuna/samplers/__init__.py	100.00% <0.00%> (ø)
optuna/visualization/_pareto_front.py	97.29% <0.00%> (ø)
optuna/distributions.py	96.20% <0.00%> (ø)
optuna/visualization/__init__.py	100.00% <0.00%> (ø)
optuna/samplers/_base.py	86.66% <0.00%> (ø)
optuna/__init__.py	100.00% <0.00%> (ø)

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[c-bata]

@contramundum53 Could you review this PR?

[toshihikoyanase]

LGTM

[contramundum53]

LGTM!