[finalize_manifests.py] Remove old stripped file before writing new one

This matters when the old stripped file lacks write permissions
because the file permissions are copied from the unstripped file
and the unstripped file (e.g. in a toolchain) might be unwritable.

TO-683 #done

Change-Id: I1eec4ed7e5f1e46917c232d5477d284a7373d593
2 files changed