sign_target_files_apks: replacing GKI signing args completely

Commit I8bd8ad3acf324931b47d45fd30bc590206b1927e adds a default
value of "gki_signing_signature_args" in the misc_info.txt for
release signing to work. However, it's better to replace the default
value entirely (e.g., --prop foo:bar) as there is no need to include
them in the final release-signed image.

Bug: 178559811
Bug: 177862434
Test: atest releasetools_test
Test: atest releasetools_py3_test
Change-Id: I060b5a7076ff3e5d883abeb7d72f3db887c9fd69