[minor] Remove unused function

[dentiny]

I found this function is unused anywhere:

[/tmp/duckdb-httpfs] (main) 
vscode@eebee01fa8d2$ git grep "SetHttpfsClientImplementation"
src/httpfs_extension.cpp:static void SetHttpfsClientImplementation(DBConfig &config, const string &value) {

[dentiny]

Looks duplicate functionality with

duckdb-httpfs/src/httpfs_extension.cpp

Lines 99 to 125 in 6b683b1

	auto callback_httpfs_client_implementation = [](ClientContext &context, SetScope scope, Value &parameter) {
	auto &config = DBConfig::GetConfig(context);
	string value = StringValue::Get(parameter);
	if (config.http_util && config.http_util->GetName() == "WasmHTTPUtils") {
	if (value == "wasm" || value == "default") {
	return;
	}
	throw InvalidInputException("Unsupported option for httpfs_client_implementation, only `wasm` and "
	"`default` are currently supported for duckdb-wasm");
	}
	#ifndef EMSCRIPTEN
	if (value == "curl") {
	if (!config.http_util || config.http_util->GetName() != "HTTPFSUtil-Curl") {
	config.http_util = make_shared_ptr<HTTPFSCurlUtil>();
	}
	return;
	}
	if (value == "httplib" || value == "default") {
	if (!config.http_util || config.http_util->GetName() != "HTTPFSUtil") {
	config.http_util = make_shared_ptr<HTTPFSUtil>();
	}
	return;
	}
	#endif
	throw InvalidInputException("Unsupported option for httpfs_client_implementation, only `curl`, `httplib` and "
	"`default` are currently supported");
	};