Remove unused var DEFAULT_MAX_CACHE_SIZE

In file: glance/image_cache/__init__.py, DEFAULT_MAX_CACHE_SIZE
is never used after declared, remove it and make glance code
tidy and pleasing.

Change-Id: I208f1dc63e2bc95bb6be94fdc88f9bfce069f67c
Merge "Remove unused var DEFAULT_MAX_CACHE_SIZE"