refactor(workhorse): remove feature flag toggle

What
---
Remove the defention of `GITLAB_WORKHORSE_EXEC` environment variable
which was introduced
gitlab-org/build/CNG!972

Why
---
We are removing the feature flag in
gitlab-org/build/CNG!986 so `exec`
is set to be used by default and `GITLAB_WORKHORSE_EXEC` no longer
changes behavior.

Reference: https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/15497


Signed-off-by: Steve Azzopardi <sazzopardi@gitlab.com>

releases/gitlab/values/values.yaml.gotmpl

@@ -891,7 +891,6 @@ gitlab:
       STACKPROF_ENABLED: "true"
       UNSTRUCTURED_RAILS_LOG: "false"
       USE_GITLAB_LOGGER: 1
-      GITLAB_WORKHORSE_EXEC: 1
   # doc/charts/gitlab/sidekiq
   sidekiq:
     enabled: {{ .Values.gitlab.sidekiq.enabled }}