Remove migration code of pip-tools legacy cache

It's not a responsibility of Prequ to remove legacy cache dir of
pip-tools.

prequ/locations.py

@@ -1,19 +1,4 @@
-import os
-from shutil import rmtree
-
-from .click import secho
 from pip.utils.appdirs import user_cache_dir
 
 # The user_cache_dir helper comes straight from pip itself
 CACHE_DIR = user_cache_dir('prequ')
-
-# NOTE
-# We used to store the cache dir under ~/.pip-tools, which is not the
-# preferred place to store caches for any platform.  This has been addressed
-# in pip-tools==1.0.5, but to be good citizens, we point this out explicitly
-# to the user when this directory is still found.
-LEGACY_CACHE_DIR = os.path.expanduser('~/.pip-tools')
-
-if os.path.exists(LEGACY_CACHE_DIR):
-    secho('Removing old cache dir {} (new cache dir is {})'.format(LEGACY_CACHE_DIR, CACHE_DIR), fg='yellow')
-    rmtree(LEGACY_CACHE_DIR)