build_image: Remove environment variable parsing

Environment variables like INSTALL_MASK is now pre-populated and hence
remove this redundant functionality.
Remove helper to fetch the image names, since the script gets this as
input.

BUG=b:233789578
TEST=CQ Passes

Cq-Depend: chromium:3684157
Change-Id: I2f433830ab51d8272907ff1338d119030cda2471
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/3683924
Tested-by: Ram Chandrasekar <rchandrasekar@google.com>
Reviewed-by: Cindy Lin <xcl@google.com>
Commit-Queue: Ram Chandrasekar <rchandrasekar@google.com>
Auto-Submit: Ram Chandrasekar <rchandrasekar@google.com>
2 files changed