chore: remove unused build dir (#1379)

# What does this PR do?
[Provide a short summary of what this PR does and why. Link to relevant
issues if applicable.]

- From old PR, it use `BUILDS_BASE_DIR` in
`llama_stack/cli/stack/configure.py`(removed).
https://github.com/meta-llama/llama-stack/pull/371/files
- Based on the current `build` code, it should only use
`DISTRIBS_BASE_DIR` to save it.

46b0a404e8/llama_stack/cli/stack/_build.py (L298)

46b0a404e8/llama_stack/cli/stack/_build.py (L301)
Pls correct me if I am understand incorrectly.
So it should no need to use in `run` now.

[//]: # (If resolving an issue, uncomment and update the line below)
[//]: # (Closes #[issue-number])

## Test Plan
[Describe the tests you ran to verify your changes with result
summaries. *Provide clear instructions so the plan can be easily
re-executed.*]

[//]: # (## Documentation)

Signed-off-by: reidliu <reid201711@gmail.com>
Co-authored-by: reidliu <reid201711@gmail.com>

llama_stack/distribution/utils/config_dirs.py

@@ -13,6 +13,4 @@ DISTRIBS_BASE_DIR = LLAMA_STACK_CONFIG_DIR / "distributions"
 
 DEFAULT_CHECKPOINT_DIR = LLAMA_STACK_CONFIG_DIR / "checkpoints"
 
-BUILDS_BASE_DIR = LLAMA_STACK_CONFIG_DIR / "builds"
-
 RUNTIME_BASE_DIR = LLAMA_STACK_CONFIG_DIR / "runtime"