Remove APEX from merged target-files package

It was a temp folder created for check_target_files_vintf.py.
But now it is wrongly archived by create_target_files_archive
in merge_target_files.py.
And then such package will cause CheckVintf exception in
ota_from_target_files.py because the APEX folder is created
again but already exists.
So use MakeTempDir to make it removed after check finished.

Test: merge_target_files & ota_from_target_files
Change-Id: Iab54afff2c346a582d5ee5228103fe091e016cd0