Workaround shutil.rmtree limitation on Windows

By default, shutil.rmtree raises an exception when deleting readonly
files on Windows.

Replace all shutil.rmtree with platform_utils.rmtree, which adds an
error handler to make files read-write when they can't be deleted.

Change-Id: I9cfea9a7b3703fb16a82cf69331540c2c179ed53
diff --git a/subcmds/gitc_delete.py b/subcmds/gitc_delete.py
index 19caac5..54f62f4 100644
--- a/subcmds/gitc_delete.py
+++ b/subcmds/gitc_delete.py
@@ -14,10 +14,10 @@
 # limitations under the License.
 
 from __future__ import print_function
-import shutil
 import sys
 
 from command import Command, GitcClientCommand
+import platform_utils
 
 from pyversion import is_python3
 if not is_python3():
@@ -50,4 +50,4 @@
       if not response == 'yes':
         print('Response was not "yes"\n Exiting...')
         sys.exit(1)
-    shutil.rmtree(self.gitc_manifest.gitc_client_dir)
+    platform_utils.rmtree(self.gitc_manifest.gitc_client_dir)