[Tools] Do not run RemoveOldSCMCheckouts() on the first checkout.

`gclient recurse' requires .gclient_entries to be present (although it does
not use it), which is not the case when Crosswalk is being checked out for
the first time. This basically made all new checkouts fail.
Merge pull request #1708 from rakuco/fix-scm-fixer

[Tools] Do not run RemoveOldSCMCheckouts() on the first checkout.