Stop purging caches in bin/storage adjust

Summary:
Fixes T6548.

  - This workflow doesn't work under reasonable configurations and isn't trivial to fix (see T6548).
  - We don't need it; this just makes things a little bit faster if you have to migrate everything (e.g., immediately after T1191) and the installs we know about have generally upgraded by now.
  - This keeps kicking PKCS8 keys out of cache which is a pain.

Test Plan: Ran `bin/storage adjust` without it doing an implicit cache purge.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6548

Differential Revision: https://secure.phabricator.com/D11377

src/infrastructure/storage/management/workflow/PhabricatorStorageManagementWorkflow.php

@@ -132,14 +132,6 @@ abstract class PhabricatorStorageManagementWorkflow
       }
     }
 
-    $console->writeOut(
-      "%s\n",
-      pht('Dropping caches, for faster migrations...'));
-
-    $root = dirname(phutil_get_library_root('phabricator'));
-    $bin = $root.'/bin/cache';
-    phutil_passthru('%s purge --purge-all', $bin);
-
     $console->writeOut(
       "%s\n",
       pht('Fixing schema issues...'));