chore(ci): remove Go agent build artifacts and variables

Delete compile_go_agent.sh and Go agent Dockerfiles which are no longer
needed since the extension was rewritten in Rust. Remove the now-orphaned
AGENT_BRANCH and AGENT_VERSION pipeline input variables from .gitlab-ci.yml.

Author: litianningdatadog

.gitlab-ci.yml

@@ -5,12 +5,6 @@ variables:
   # This will be overwritten by the tag version if it is a release.
   VERSION: dev
   # Manual trigger variables
-  AGENT_BRANCH:
-    description: "Branch of the datadog-agent repository to use."
-    value: main
-  AGENT_VERSION:
-    description: "Latest release version of the datadog-agent to tag the build with."
-    value: "7.66.0"
   PIPELINE_LAYER_SUFFIX:
     description: "Suffix to be appended to the layer name (default empty)."
     value: ""