chore: remove requirements.txt

We use pyprojects to maintain our dependencies. Additionally, we use a
uv.lock to pin the dependencies versions.
Having a requirements.txt should not be needed. Also, I think this
prevents dependabot from updating our uv.lock.

Before this https://github.com/meta-llama/llama-stack/issues/2908 gets
implemented, let's see if we can at least get some update from
pyproject.

Signed-off-by: Sébastien Han <seb@redhat.com>

.pre-commit-config.yaml

@@ -61,8 +61,7 @@ repos:
             "--frozen",
             "--no-hashes",
             "--no-emit-project",
-            "--no-default-groups",
-            "--output-file=requirements.txt"
+            "--no-default-groups"
         ]
 
 -   repo: https://github.com/pre-commit/mirrors-mypy