Removing double delete from flashall

From aosp/2475604 the functionality of deleting retrofit partitions
wasn't added back correctly. aosp/837186 adds this logic in. We should
only have one delete that exists if the image is secondary and is
dynamic.

Unfortunately don't have a retrofit device to test on, but from an eye
test this logic seems to match the old functionality and should be
working

Test: m fastboot
Change-Id: I5481893ab1638541d21813efe1c4aab5219e1dcd