Remove a bool type assignment in lr_finder.py (#13652)

Author: donlapark

src/pytorch_lightning/tuner/lr_finder.py

@@ -39,7 +39,7 @@
 else:
     from tqdm import tqdm
 
-_MATPLOTLIB_AVAILABLE: bool = _RequirementAvailable("matplotlib")  # type: ignore[assignment]
+_MATPLOTLIB_AVAILABLE = _RequirementAvailable("matplotlib")
 if _MATPLOTLIB_AVAILABLE and TYPE_CHECKING:
     import matplotlib.pyplot as plt