feat(db/redis)!: remove backward compatibility code

[MaineK00n]

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What did you implement:

In #187, CPE#Cache#Titles is no longer used, and CPE#Titles has been changed from a SET type to a String type.
Once the changes have fully taken hold, any code that is not needed will be removed at an appropriate time for backward compatibility.

Type of change

• Breaking change (fix or feature that would cause existing functionality to not work as expected)

How Has This Been Tested?

Checklist:

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• Write tests
• Write documentation
• Check that there aren't other open pull requests for the same issue/feature
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Is this ready for review?: YES

Reference

[shino]

Perfect!