[llvm] [llvm-objdump] Remove some unneeded headers. (PR #120541)

[James Henderson via llvm-commits]

jh7370 wrote:

> Almost all headers I removed are included by the relative header. For example, StringSet is included by llvm-objdump.h. It’s the first time I submit my patch to community, sorry if I did something wrong🥺.

Don't worry, you've done nothing wrong! I just needed to understand the context to be able to focus my review appropriately. I'll try to look at this in the next couple of workdays, but I've got a couple of higher priority items to look at before Christmas, so may not get to it. Somebody else is welcome to review it instead, if they have time!

https://github.com/llvm/llvm-project/pull/120541