Remove unused DEFAULT_FORMAT constant

Signed-off-by: Srikanth Padakanti <srikanth_padakanti@apple.com>

Author: srikanthpadakanti

data-prepper-plugins/file-source/src/main/java/org/opensearch/dataprepper/plugins/source/file/FileSourceConfig.java

@@ -27,7 +27,6 @@ public class FileSourceConfig {
     static final String ATTRIBUTE_FORMAT = "format";
     static final int DEFAULT_TIMEOUT = 5_000;
     static final String DEFAULT_TYPE = "string";
-    static final String DEFAULT_FORMAT = "plain";
     static final String EVENT_TYPE = "event";
 
     @JsonProperty(ATTRIBUTE_PATH)