Remove the key parameter when verifying avb images in validate_target_files

If a key is specified, the avbtool always use the input key to verify
all the chained images. And this will cause failures when the vbmeta &
system use different keys (e.g. RSA 4096 vs RSA2048). Because the public
key to vbmeta will always fail to verify the system image. Remove the
'--key' parameter in the verification command, so the avbtool will use
the embedded public in the image.

Test: validate target-file from sdk_gphone_x86_64
Bug: 148916990
Change-Id: I9d31be0f8c32af605af94fa73d07818f40f51ec4