Remove archive/stat_unsupported.go because it is not used

LUtimesNano and all other functions were implemented on pkg/system after
d6114c0da0e844199e3d23c60a04434566fb5392.

Docker-DCO-1.1-Signed-off-by: Kato Kazuyoshi <kato.kazuyoshi@gmail.com> (github: kzys)
intermediate image layers are used for more than the build

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@fosiki.com> (github: SvenDowideit)
Merge pull request #5051 from SvenDowideit/build-rm-help

intermediate image layers are used for more than the build
Merge pull request #5038 from kzys/remove-archive-stat

Remove archive/stat_unsupported.go because it is not used