build: remove the Exercise.async field

The stage1 C++ compiler is gone forever.

Remove the custom support and documentation for the old stage1 compiler
in build.zig and README.md.

README.md

@@ -74,9 +74,6 @@ Once you have a build of the Zig compiler that works with Ziglings, they'll
 continue to work together. But keep in mind that if you update one, you may
 need to also update the other.
 
-Also note that the current "stage 1" Zig compiler is very strict
-about input: 
-[no tab characters or Windows CR/LF newlines are allowed](https://github.com/ziglang/zig/issues/544).
 
 ### Version Changes
 

build.zig

@@ -29,10 +29,6 @@ pub const Exercise = struct {
     /// Set this to true to check stdout instead.
     check_stdout: bool = false,
 
-    /// This exercise makes use of the async feature.
-    /// We need to keep track of this, so we compile without the self hosted compiler
-    @"async": bool = false,
-
     /// This exercise makes use of C functions
     /// We need to keep track of this, so we compile with libc
     C: bool = false,
@@ -356,12 +352,6 @@ const ZiglingStep = struct {
         zig_args.append(builder.zig_exe) catch unreachable;
         zig_args.append("build-exe") catch unreachable;
 
-        // Enable the stage 1 compiler if using the async feature
-        // disabled because of https://github.com/ratfactor/ziglings/issues/163
-        // if (self.exercise.@"async") {
-        //     zig_args.append("-fstage1") catch unreachable;
-        // }
-
         // Enable C support for exercises that use C functions
         if (self.exercise.C) {
             zig_args.append("-lc") catch unreachable;
@@ -1005,49 +995,41 @@ const exercises = [_]Exercise{
         .main_file = "084_async.zig",
         .output = "foo() A",
         .hint = "Read the facts. Use the facts.",
-        .@"async" = true,
         .skip = true,
     },
     .{
         .main_file = "085_async2.zig",
         .output = "Hello async!",
-        .@"async" = true,
         .skip = true,
     },
     .{
         .main_file = "086_async3.zig",
         .output = "5 4 3 2 1",
-        .@"async" = true,
         .skip = true,
     },
     .{
         .main_file = "087_async4.zig",
         .output = "1 2 3 4 5",
-        .@"async" = true,
         .skip = true,
     },
     .{
         .main_file = "088_async5.zig",
         .output = "Example Title.",
-        .@"async" = true,
         .skip = true,
     },
     .{
         .main_file = "089_async6.zig",
         .output = ".com: Example Title, .org: Example Title.",
-        .@"async" = true,
         .skip = true,
     },
     .{
         .main_file = "090_async7.zig",
         .output = "beef? BEEF!",
-        .@"async" = true,
         .skip = true,
     },
     .{
         .main_file = "091_async8.zig",
         .output = "ABCDEF",
-        .@"async" = true,
         .skip = true,
     },