Revert "Fix the default value of the "doma.entity.field.prefix" option"

This reverts commit 1c25457.

Author: nakamura-to

doma-processor/src/main/java/org/seasar/doma/internal/apt/Options.java

@@ -192,7 +192,7 @@ public static class Constants {
 
     public static final String DEFAULT_DAO_SUFFIX = "Impl";
 
-    public static final String DEFAULT_ENTITY_FIELD_PREFIX = "";
+    public static final String DEFAULT_ENTITY_FIELD_PREFIX = "none";
 
     public static final String DEFAULT_CONFIG_PATH = "doma.compile.config";