clean(autogpt): Remove old plugin system (#7097)

### Background

Follow up after merging https://github.com/Significant-Gravitas/AutoGPT/pull/7054, old plugins will no longer be used.

### Changes 🏗️

- Removed all dead code needed to load and use plugins.
- Removed `auto-gpt-plugin-template` dependency
- Removed `rev=` from `autogpt-forge` dependency (the set `rev` had incompatible `duckduckgo-search` versions)
- Kept `--install-plugin-deps` CLI option and dead code associated (may be needed for new plugins)

docs/content/AutoGPT/configuration/options.md

@@ -33,7 +33,6 @@ Configuration is controlled through the `Config` object. You can set configurati
 - `OPENAI_API_KEY`: *REQUIRED*- Your [OpenAI API Key](https://platform.openai.com/account/api-keys).
 - `OPENAI_ORGANIZATION`: Organization ID in OpenAI. Optional.
 - `PLAIN_OUTPUT`: Plain output, which disables the spinner. Default: False
-- `PLUGINS_CONFIG_FILE`: Path of the Plugins Config file relative to the AutoGPT root directory. Default: plugins_config.yaml
 - `PROMPT_SETTINGS_FILE`: Location of the Prompt Settings file relative to the AutoGPT root directory. Default: prompt_settings.yaml
 - `REDIS_HOST`: Redis Host. Default: localhost
 - `REDIS_PASSWORD`: Redis Password. Optional. Default: