imagebuildah/executor.go: fix gosimple warning
imagebuildah/executor.go: fix gosimple warning

Fix the following warning:

> 	imagebuildah/executor.go:307:6: S1033: unnecessary guard around call to delete (gosimple)
>						if _, stillUnused := exec.unusedArgs[list[0]]; stillUnused {
>						^

Signed-off-by: Kir Kolyshkin <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>

TomSweeneyRedHat/buildah,nalind/buildah-1,rhatdan/buildah,projectatomic/buildah,TomSweeneyRedHat/buildah,projectatomic/buildah,rhatdan/buildah,nalind/buildah-1,rhatdan/buildah,nalind/buildah-1,TomSweeneyRedHat/buildah,projectatomic/buildah