Only import Field when single-value httpHeader is actually in the model

Author: haydenbaker

codegen/core/src/main/java/software/amazon/smithy/python/codegen/integrations/HttpBindingProtocolGenerator.java

@@ -199,7 +199,7 @@ private void serializeHeaders(
     ) {
         writer.pushState(new SerializeFieldsSection(operation));
         writer.addDependency(SmithyPythonDependency.SMITHY_HTTP);
-        writer.addImports("smithy_http", Set.of("Field", "Fields"));
+        writer.addImport("smithy_http", "Fields");
         writer.write("""
                 headers = Fields(
                     [
@@ -331,6 +331,7 @@ private void serializeIndividualHeaders(GenerationContext context, PythonWriter
                             headers.extend(tuples_to_fields(($S, $L) for e in input.$L$L))
                             """, binding.getLocationName(), inputValue, pythonName, trailer);
                 } else {
+                    writer.addImport("smithy_http", "Field");
                     var dataSource = "input." + pythonName;
                     var inputValue = target.accept(new HttpMemberSerVisitor(
                             context,