Fix how we delete the symlink

The previous way of checking the file existance would be fooled
if the symlink exists but the target doesn't.

Flag: EXEMPT host test change only
Fix: 369687596
Test: manual
    $ rm /tmp/Ravenwood-stats_CtsOsTestCasesRavenwood_2*.csv # remove only the target
    $ atest CtsOsTestCasesRavenwood

Change-Id: I84c94ca5913182a06422f71743a3b809ab805fda