wic: Make sure file exists before removing it

wic: Make sure file exists before removing it

Bunch of os.remove calls were added to the partition.py lately.
They're causing wic to fail with OSError: [Errno 2] No such file or directory
if file doesn't exist.

Added check for file existence to all recently added calls of
os.remove. That should fix this regression.

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>