Remove unused import from SRX.java

The `Collectors` class from `java.util.stream` was imported but not used in the code.

Author: miurahr

src/main/java/org/omegat/core/segmentation/SRX.java

@@ -59,7 +59,6 @@
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
-import java.util.stream.Collectors;
 
 /**
  * The class with all the segmentation data possible -- rules, languages, etc.