Remove unecessary commentary in PD901 message (#8625)
## Summary
Removes unnecessary commentary from the PD901 message. This does make it
different from pandas-vet, but it improves consistency with the rest of
messages.
Current Message:
> `df` is a bad variable name. Be kinder to your future self.
New Message
> `df` is a bad variable name.
## Test Plan
The relevant snapshot has been updated with the new message.
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Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>
