Remove Dynatrace OneAgent Download

Previously, the Dynatrace OneAgent agent had to be downloaded from the
Internet.  More recently, this agent is downloaded from the OneAgent server
directly, and it no longer must be packaged within the buildpack.  This change
removes the configuration that caused it to be downloaded.

[resolves #764]

Signed-off-by: Ben Hale <[email protected]>

Author: nebhale