docker-util: Remove building of GCC 5 and 6 Docker images

Building of these images is no longer required as GCC 5 and 6 have been
dropped.

Change-Id: Ife0331843214fe545c5620a2f77ede3adb436700
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/47462
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
1 file changed