CLN: remove unused c_encoding
`c_encoding` has always been `NULL` since #36997. This allows removing/simplifying a lot of code in pandas/_libs/parsers.pyx.
nice @twoertwein 

any perf gains from this? > any perf gains from this?

I didn't check, but I wouldn't expect a significant change as it mostly removes dead code. I can run the benchmarks later today.