Remove unused headers

We might have around 2000 unused headers in our codebase, this might increase compile time unnecessarily. We should consider removing this.

One idea is generally commenting out an include line to see if the code still compiles but this should be done for pg11, pg12 and pg13.

For example I quickly tried something like this for pg13 and it worked but should consider pg11 and pg12 as well. https://github.com/citusdata/citus/tree/remove/unusedHeaders