Serialize MediaType as an integer to avoid string parsing

Resolves opensearch-project#15979

Signed-off-by: Andrew Ross <andrross@amazon.com>

libs/x-content/src/main/java/org/opensearch/common/xcontent/XContentType.java

@@ -239,7 +239,7 @@ public XContentBuilder contentBuilder(final OutputStream os) throws IOException
         }
     };
 
-    private int index;
+    private final int index;
 
     XContentType(int index) {
         this.index = index;
@@ -259,15 +259,6 @@ public String format() {
         return subtype();
     }
 
-    @Override
-    public void writeTo(StreamOutput output) throws IOException {
-        if (output.getVersion().onOrAfter(Version.V_3_0_0)) {
-            output.writeVInt(uniqueId());
-        } else {
-            output.writeString(this.mediaType());
-        }
-    }
-
     @Override
     public int uniqueId() {
         return this.ordinal();