chore: s.remove_staging_dirs() should only be called once (#362)

There is [an issue](https://github.com/googleapis/python-firestore/blob/master/owlbot.py#L60) in the `owlbot.py` file added in #352 in that [s.remove_staging_dirs()](https://github.com/googleapis/synthtool/blob/master/synthtool/transforms.py#L309) should only be called once after all the files are copied over.  [get_staging_dirs()](https://github.com/googleapis/synthtool/blob/master/synthtool/transforms.py#L280) will only return staging directories that exist.

owlbot.py

@@ -57,8 +57,6 @@ def update_fixup_scripts(library):
         update_fixup_scripts(library)
         s.move(library / "scripts")
 
-s.remove_staging_dirs()
-
 for library in s.get_staging_dirs(admin_default_version):
     if library.parent.absolute() == 'admin':
         s.move(
@@ -70,8 +68,6 @@ def update_fixup_scripts(library):
         update_fixup_scripts(library)
         s.move(library / "scripts")
 
-s.remove_staging_dirs()
-
 for library in s.get_staging_dirs(bundle_default_version):
     if library.parent.absolute() == 'bundle':
         s.replace(
@@ -251,4 +247,4 @@ def lint_setup_py(session):
 
 *************
 Test Coverage"""
-)
+)