Remove a bool type assignment in lr_finder.py

[donlapark]

What does this PR do?

Remove a bool type assignment of _MATPLOTLIB_AVAILABLE in pytorch_lightning/tuner/lr_finder.py. This is a follow-up from #13513 and #13445.

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[donlapark]

Thanks everyone for the approvals!

There is still one problem though: the Azure Pipelines / pytorch-lightning (GPUs) (testing PyTorch - stable) check is taking an abnormally long time.