11917: Do not clear rails cache at boot time.

This seems like a good idea when there is only one process that has
access to the cache. However, in real life the crunch1 dispatcher,
rake tasks, and the apiserver all share the cache, and they don't all
start up at the same time. This means clear() can run while other
operations like fetch() are in progress, which is apparently not a
supported use. With the Rails file cache, if clear() removes a
directory at an inopportune moment, fetch() crashes (see
https://dev.arvados.org/issues/11917).

Arvados-DCO-1.1-Signed-off-by: Tom Clegg <tom@curoverse.com>