core: remove mustache in extended deps (#27629)

Remove mustache from extended deps -- we vendor the mustache
implementation

libs/core/extended_testing_deps.txt

@@ -1,2 +1 @@
 jinja2>=3,<4
-mustache>=0.1.4,<1

libs/core/tests/unit_tests/prompts/test_chat.py

@@ -328,7 +328,6 @@ def test_chat_prompt_template_from_messages_jinja2() -> None:
 
 
 @pytest.mark.requires("jinja2")
-@pytest.mark.requires("mustache")
 @pytest.mark.parametrize(
     "template_format,image_type_placeholder,image_data_placeholder",
     [