chore(ci): remove Go agent build artifacts and variables (#1052)

## Summary
- Delete `compile_go_agent.sh` and Go agent Dockerfiles
(`Dockerfile.go_agent.compile`, `Dockerfile.go_agent.alpine.compile`)
which are no longer needed since the extension was rewritten in Rust
(bottlecap)
- Remove the now-orphaned `AGENT_BRANCH` and `AGENT_VERSION` pipeline
input variables from `.gitlab-ci.yml`

## Jira
[SVLS-8636](https://datadoghq.atlassian.net/browse/SVLS-8636)

[SVLS-8636]:
https://datadoghq.atlassian.net/browse/SVLS-8636?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

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: ""