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!

epi/Cargo.toml

@@ -29,5 +29,4 @@ serde = { version = "1", optional = true }
 
 [features]
 default = []
-http = []
 persistence = ["ron", "serde"]