remove parallel() processing as per discussion in aws/serverless-java-container#1089 (comment)

This reverts commit 1785eb57c7d72f6c1b8468f2e3ad13ddb8298589.

Author: nicollas1020

aws-serverless-java-container-core/src/main/java/com/amazonaws/serverless/proxy/internal/servlet/AwsHttpServletRequest.java

@@ -597,7 +597,7 @@ protected Map<String, String[]> generateParameterMap(MultiValuedTreeMap<String,
             queryStringParams = decodedQs;
         } else {
             // If it's case insensitive, we check the entire map on every parameter
-            queryStringParams = decodedQs.entrySet().stream().parallel().collect(
+            queryStringParams = decodedQs.entrySet().stream().collect(
                 Collectors.toMap(
                     Map.Entry::getKey,
                     e -> getQueryParamValuesAsList(decodedQs, e.getKey(), false)