Replace all http code in epi/eframe/egui_glium/egui_web with ehttp (#697)

I've extracted all the http request code and turned it
into its own crate at <https://github.com/emilk/ehttp>.

There was never a reason for the HTTP request library to be part of
`eframe`. Much better to have it as its own crate!

egui_web/src/lib.rs

@@ -22,8 +22,6 @@
 #![warn(clippy::all, rust_2018_idioms)]
 
 pub mod backend;
-#[cfg(feature = "http")]
-pub mod http;
 mod painter;
 pub mod screen_reader;
 pub mod webgl1;