Remove setting Redis config file path via env var

What does this MR do and why?

Resolves #388255 (closed)

Remove setting Redis config file path via env var

Setting environment variable like GITLAB_REDIS_CACHE_CONFIG_FILE and the global GITLAB_REDIS_CONFIG_FILE for Redis config file path is no longer supported.

Changelog: removed

How to set up and validate locally

1. Copy the existing config/redis.cache.yml to a new file:

   ❯ cp config/redis.cache.yml config/from_env_var.yml

2. Edit config/from_env_var.yml to use another db name, e.g.:

   ❯ cat config/from_env_var.yml
   ---
   development: unix:/Users/gregoriusmarco/Documents/workspace/gdk-10-22/redis/redis.socket?db=1
   test: unix:/Users/gregoriusmarco/Documents/workspace/gdk-10-22/redis/redis.socket?db=12

3. Start Rails console with the environment variable pointing to the file path:

   GITLAB_REDIS_CACHE_CONFIG_FILE=/Users/gregoriusmarco/Documents/workspace/gdk-10-22/gitlab/config/from_env_var.yml gdk rails c

4. Check that Gitlab::Redis::Cache is still pointing to the config file derived from the name:

   [1] pry(main)> Gitlab::Redis::Cache.config_file_name
   => "/Users/gregoriusmarco/Documents/workspace/gdk-10-22/gitlab/config/redis.cache.yml"
   [2] pry(main)> Gitlab::Redis::Cache.url
   => "unix:/Users/gregoriusmarco/Documents/workspace/gdk-10-22/redis/redis.socket?db=2"

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