Remove syntax redundant with callconv

[tadeokondrak]

Changes:

• extern fn is a property of a function definition, not a prototype (did not have auto-fixes, now does)

extern fn foo() void {} // valid
extern extern fn foo() void {} // no longer valid
const bar = fn () callconv(.C) void; // valid
const baz = extern fn () void; // no longer valid

• async fn is gone (did not have auto-fixes, now does)

fn foo() void { suspend; } // valid
fn bar() callconv(.Async) void {} // valid
async fn baz() void {} // no longer valid

#3977 (comment)

19:34 <tdeo> so `async fn` does still exist in the language, is it redundant and can be removed now?
19:35 <andrewrk> tdeo, yes it should be removed in favor of callconv(.Async)

• stdcallcc and nakedcc have been removed from the self-hosted parser, they were already gone from the C++ version (previously had auto-fixes, now removed)

[andrewrk]

Nice work!