download_gki: Clean up unnecessary code and files

This patch handles a variety of small cleanups to download_gki:
* Removes GKI-files and replaces it with an array in download_gki itself
* Removes the conditional check for downloading the -user images
* Only unzips the chosen boot.img type from the img.zip
* Remove some string redundancies

Bug: 194513402
Signed-off-by: J. Avila <elavila@google.com>
Change-Id: Ifd4cb7b2a6e1de44e493f68f6f80712ee16d880c