Remove unstable --pretty flag

It doesn't do anything `--unpretty` doesn't, and due to a bug, also
didn't show up in `--help`. I don't think there's any reason to keep it
around, I haven't seen anyone using it.

src/tools/compiletest/src/header.rs

@@ -306,7 +306,7 @@ pub struct TestProps {
     // a proc-macro and needs `#![crate_type = "proc-macro"]`. This ensures
     // that the aux file is compiled as a `proc-macro` and not as a `dylib`.
     pub no_prefer_dynamic: bool,
-    // Run --pretty expanded when running pretty printing tests
+    // Run -Zunpretty expanded when running pretty printing tests
     pub pretty_expanded: bool,
     // Which pretty mode are we testing with, default to 'normal'
     pub pretty_mode: String,

src/tools/compiletest/src/runtest.rs

@@ -599,7 +599,7 @@ impl<'test> TestCx<'test> {
             return;
         }
 
-        // additionally, run `--pretty expanded` and try to build it.
+        // additionally, run `-Zunpretty=expanded` and try to build it.
         let proc_res = self.print_source(ReadFrom::Path, "expanded");
         if !proc_res.status.success() {
             self.fatal_proc_rec("pretty-printing (expanded) failed", &proc_res);