header_timeout is documented, but does nothing (since 2014)

See 863c1b35905234a43021bf0b0fd2adceb1fc4ef6.

(Please, remove it from [here](https://github.com/tarantool/http/blob/f11a68ffe49a3c4baf03f23df929afd6ed5f5588/http/server.lua#L1316) too.)