docs: remove spurious main functions

Author: midgleyc

compiler/rustc_error_codes/src/error_codes/E0722.md

@@ -7,8 +7,6 @@ Erroneous code example:
 
 #[optimize(something)] // error: invalid argument
 pub fn something() {}
-
-fn main() {}
 ```
 
 The `#[optimize]` attribute should be used as follows:
@@ -25,8 +23,6 @@ For example:
 
 #[optimize(size)]
 pub fn something() {}
-
-fn main() {}
 ```
 
 See [RFC 2412] for more details.