[NFC] Remove the llvm-external-projects/iree-dialects directory (#24466)#24612

Alex-Wengg wants to merge 2 commits into

iree-org:mainiree-org/iree:mainfrom

Alex-Wengg:retire-iree-dialects-finalAlex-Wengg/iree:retire-iree-dialects-finalCopy head branch name to clipboard