Revert "Use rustdoc --document-private-item instead of deprecated flags"

This reverts commit d96f0ff6a7.

python/servo/testing_commands.py

@@ -255,12 +255,9 @@ class MachCommands(CommandBase):
 
         packages.discard('stylo')
 
-        env = self.build_env()
+        env = self.build_env(test_unit=True)
         env["RUST_BACKTRACE"] = "1"
 
-        # Work around https://github.com/rust-lang/cargo/issues/4790
-        del env["RUSTDOCFLAGS"]
-
         if "msvc" in host_triple():
             # on MSVC, we need some DLLs in the path. They were copied
             # in to the servo.exe build dir, so just point PATH to that.