Remove prompt templates from GenAI config

[kunal-vaishnavi]

Description

This PR removes generating prompt templates and storing them in the GenAI config.

Motivation and Context

The prompt templates are not used internally anymore.

[baijumeswani]

Shall we remove the prompt processing code as well:

onnxruntime-genai/src/config.cpp

Lines 592 to 622 in 89a2889

	struct PromptTemplates_Element : JSON::Element {
	explicit PromptTemplates_Element(std::optional<Config::Model::PromptTemplates>& v) : v_{v} {}
	void OnValue(std::string_view name, JSON::Value value) override {
	// if one of templates is given in json, then any non-specified template will be default "{Content}"
	if (name == "assistant") {
	EnsureAvailable();
	v_->assistant = JSON::Get<std::string_view>(value);
	} else if (name == "prompt") {
	EnsureAvailable();
	v_->prompt = JSON::Get<std::string_view>(value);
	} else if (name == "system") {
	EnsureAvailable();
	v_->system = JSON::Get<std::string_view>(value);
	} else if (name == "user") {
	EnsureAvailable();
	v_->user = JSON::Get<std::string_view>(value);
	} else {
	throw JSON::unknown_value_error{};
	}
	}
	private:
	std::optional<Config::Model::PromptTemplates>& v_;
	void EnsureAvailable() {
	if (!v_.has_value()) {
	v_.emplace();
	}
	}
	};