Mozilla rustc check does not support crossbuild: let's remove it
Remove calls to unwrap_rustup, they fail if rustup isn't present

--- build/moz.configure/rust.configure	2019-07-06 03:48:30.000000000 +0200
+++ build/moz.configure/rust.configure	2019-07-09 22:35:23.326851798 +0200
@@ -79,9 +79,6 @@
 
     return unwrap
 
-rustc = unwrap_rustup(rustc, 'rustc')
-cargo = unwrap_rustup(cargo, 'cargo')
-
 
 set_config('CARGO', cargo)
 set_config('RUSTC', rustc)
@@ -294,26 +291,6 @@
 
             os.write(in_fd, source)
             os.close(in_fd)
-
-            cmd = [
-                rustc,
-                '--crate-type', 'staticlib',
-                target_arg,
-                '-o', out_path,
-                in_path,
-            ]
-
-            def failed():
-                die(dedent('''\
-                Cannot compile for {} with {}
-                The target may be unsupported, or you may not have
-                a rust std library for that target installed. Try:
-
-                  rustup target add {}
-                '''.format(host_or_target.alias, rustc, rustc_target.alias)))
-            check_cmd_output(*cmd, onerror=failed)
-            if not os.path.exists(out_path) or os.path.getsize(out_path) == 0:
-                failed()
         finally:
             os.remove(in_path)
             os.remove(out_path)