[llvm] [unused-includes][ADT] Don't include DJB.h as it's not used. NFC. (PR #88211)

[Volodymyr Sapsai via llvm-commits]

vsapsai wrote:

> Thanks for the catch! Must've been a holdover from the original version of the patch.

Thanks for taking a look that's not an upcoming use of DJB hashing.

https://github.com/llvm/llvm-project/pull/88211