Removed a clear_cache statement in contrib.sites.create_default_site.

It was originally added to fix a test (refs #7514); but Site now has a
pre_save signal handler (refs #19698) to clear the cache which makes
this call redundant.
Fixed #23929 -- Added more tests for create_default_site.

Thanks Tim Graham for all the reviewing.

Refs: #15346, #15573, #16353, #16828.