Remove non-constant input check from ConvActivationFusion

[edgchen1]

Description

An additional check for non-constant inputs was added to ConvActivationFusion in #20282. This was to avoid fusing an Add in a Conv+Add+Relu that has another non-constant input.

onnxruntime/onnxruntime/core/optimizer/conv_activation_fusion.cc

Lines 26 to 39 in 6c8cb6a

	const Node* next_node = &*node.OutputNodesBegin();
	// ensure that the target node also has only one input that is not an initializer
	const size_t input_edges_total = next_node->GetInputEdgesCount();
	int non_const_edges = 0;
	for (size_t edge_idx = 0; edge_idx < input_edges_total; ++edge_idx) {
	if (!graph_utils::NodeArgIsConstant(graph_viewer.GetGraph(), *next_node->InputDefs()[edge_idx])) {
	++non_const_edges;
	}
	}
	if (non_const_edges > 1) {
	return nullptr;
	} else {
	return next_node;
	}

However, this check fails to account for implicit inputs and will read past the end of a node's explicit input defs if any implicit inputs are present.

Moreover, this check is no longer necessary after #19470 removed Conv+Add+Relu fusion from ConvActivationFusion.

This change removes the check and some other unused code.

Motivation and Context

Fix #24473.