fix: remove hard-coded overwrite_files in AzureStorage

The overwrite_files is True by default, and should be settable from the
django settings using AZURE_OVERWRITE_FILES.

Removing this override will keep the default behaviour (True), while
still allowing users to change it.

NOTE: this change is important because django / django-storages changed
the semantics of the .exists() function, which now returns whether a
path is free for upload (and not whether the file already exists on the
remote storage). With overwrite_files set to True, .exists() will now
always return False.

See jschneier/django-storages#1430

django_storage_url/backends/az.py

@@ -56,7 +56,6 @@ def __init__(self, dsn):
  self.azure_container = container_name
  self.azure_ssl = secure_urls
  self.max_memory_size = 10 * 1024**2
- self.overwrite_files = True
  self.location = ""
  self.base_url = str(base_url)