Don't remove existing explicitly stashed blocks

When automatically stashing overlapping blocks, should the stash
file already exist due to an explicit stash command, it's not safe
to remove the stash file after the command has completed.

Note that it is safe to assume that the stash file will remain in
place during the execution of the next command, so we don't have
take other measures to preserve overlapping blocks.

The stash file itself will be removed by a free command when it's
no longer needed.

Bug: 20297065
Change-Id: I8ff1a798b94086adff183c5aac03260eb947ae2c