Fix T91979: Don't mark string nodes as supporting fields

Even though these nodes do support fields internally, there are no
input string sockets that support fields currently, so removing
the diamond sockets for now helps avoid confusion.

Differential Revision: https://developer.blender.org/D12828

source/blender/nodes/function/nodes/node_fn_string_length.cc

@@ -24,7 +24,6 @@ namespace blender::nodes {
 
 static void fn_node_string_length_declare(NodeDeclarationBuilder &b)
 {
-  b.is_function_node();
   b.add_input<decl::String>("String");
   b.add_output<decl::Int>("Length");
 };

source/blender/nodes/function/nodes/node_fn_string_substring.cc

@@ -22,7 +22,6 @@ namespace blender::nodes {
 
 static void fn_node_string_substring_declare(NodeDeclarationBuilder &b)
 {
-  b.is_function_node();
   b.add_input<decl::String>("String");
   b.add_input<decl::Int>("Position");
   b.add_input<decl::Int>("Length").min(0);

source/blender/nodes/function/nodes/node_fn_value_to_string.cc

@@ -21,7 +21,6 @@ namespace blender::nodes {
 
 static void fn_node_value_to_string_declare(NodeDeclarationBuilder &b)
 {
-  b.is_function_node();
   b.add_input<decl::Float>("Value");
   b.add_input<decl::Int>("Decimals").min(0);
   b.add_output<decl::String>("String");