Bug 1323557 - preserve vendored crates when re-vendoring; r=ted.mielczarek

`mach vendor rust` currently removes third_party/rust before `cargo update`,
which prevents modifying vendored crates for local testing and try pushes.
It's also unnecessary.  So this patch removes the code that removes the dir.

MozReview-Commit-ID: IE0FZ3of8Py

--HG--
extra : rebase_source : 5236e2decd95f4c83b34430d5423b80e84acdf07

python/mozbuild/mozbuild/vendor_rust.py

@@ -289,14 +289,12 @@ license file's hash.
 
         relative_vendor_dir = 'third_party/rust'
         vendor_dir = mozpath.join(self.topsrcdir, relative_vendor_dir)
-        self.log(logging.INFO, 'rm_vendor_dir', {}, 'rm -rf %s' % vendor_dir)
-        mozfile.remove(vendor_dir)
 
         # We use check_call instead of mozprocess to ensure errors are displayed.
         # We do an |update -p| here to regenerate the Cargo.lock file with minimal changes. See bug 1324462
         subprocess.check_call([cargo, 'update', '-p', 'gkrust'], cwd=self.topsrcdir)
 
-        subprocess.check_call([cargo, 'vendor', '--quiet', '--no-delete', '--sync', 'Cargo.lock'] + [vendor_dir], cwd=self.topsrcdir)
+        subprocess.check_call([cargo, 'vendor', '--quiet', '--sync', 'Cargo.lock'] + [vendor_dir], cwd=self.topsrcdir)
 
         if not self._check_licenses(vendor_dir):
             self.log(logging.ERROR, 'license_check_failed', {},