Drop native offline chat support with llama-cpp-python

It is recommended to chat with open-source models by running an
open-source server like Ollama, Llama.cpp on your GPU powered machine
or use a commercial provider of open-source models like DeepInfra or
OpenRouter.

These chat model serving options provide a mature Openai compatible
API that already works with Khoj.

Directly using offline chat models only worked reasonably with pip
install on a machine with GPU. Docker setup of khoj had trouble with
accessing GPU. And without GPU access offline chat is too slow.

Deprecating support for an offline chat provider directly from within
Khoj will reduce code complexity and increase developement velocity.
Offline models are subsumed to use existing Openai ai model provider.

.devcontainer/Dockerfile

@@ -18,8 +18,8 @@ ENV PATH="/opt/venv/bin:${PATH}"
 COPY pyproject.toml README.md ./
 
 # Setup python environment
-    # Use the pre-built llama-cpp-python, torch cpu wheel
-ENV PIP_EXTRA_INDEX_URL="https://download.pytorch.org/whl/cpu https://abetlen.github.io/llama-cpp-python/whl/cpu" \
+    # Use the pre-built torch cpu wheel
+ENV PIP_EXTRA_INDEX_URL="https://download.pytorch.org/whl/cpu" \
     # Avoid downloading unused cuda specific python packages
     CUDA_VISIBLE_DEVICES="" \
     # Use static version to build app without git dependency