[clang-tools-extra] [clang-doc] remove FullName from serialization (PR #166595)

[Paul Kirth via cfe-commits]

https://github.com/ilovepi approved this pull request.

Surprised this is dead. I thought the HTML still used it, but if tests are passing, then its probably OK.

https://github.com/llvm/llvm-project/pull/166595