Replace generated NGINX ingress controller

Removes all references of the NGINX ingress controller in the GitLab Operator controller code, as it will be deployed via the OperatorHub (or test.sh) per our docs.

This helps to decouple our GitLab Operator from a specific Ingress controller, and should make future changes/upgrades easier by modifying the YAML / docs rather than the codebase.

Related to https://gitlab.com/gitlab-org/gl-openshift/gitlab-operator/-/issues/105

Test instructions

1. Prepare an OpenShift cluster (see our docs in this repo)

2. Modify config/samples/apps_v1beta1_gitlab.yaml with your domain:

   apiVersion: apps.gitlab.com/v1beta1
   kind: GitLab
   metadata:
     name: gitlab
   spec:
     chart:
       version: CHART_VERSION
       values: # added global.hosts.domain below
         global:
           hosts:
             domain: yourdomain.com

3. TAG=ab5c7d86 CLEANUP=no ./scripts/test.sh

4. Navigate to gitlab.yourdomain.com in the browser and log in!