Revert "build-acs: adapt to changes in acs v19.01_REL2.1"

This reverts commit 58ee8a56. ACS REL2.1
is not working correctly on Arm's enterprise reference platforms. So use
REL1.7 of ACS and keep this patch reverted until issues in REV2.1 are
resolved.

Change-Id: I0659d5c5361578ca37a551102086296b2193e4d3
Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>