generate_metadata.py: remove metadata.bin if it already exits

While generating the metadata.bin, it is opened multiple times within
the script in the append mode.  Hence before generating a new
metadata.bin, the old file has to be deleted to avoid appending to it.

Signed-off-by: Davidson K <davidson.kumaresan@arm.com>
Change-Id: Ic0ab22c7465c939a590d3b7e27a394422d4e6b72