fix warnings for non-thrown IOExceptions

com.oracle.truffle.r.engine/src/com/oracle/truffle/r/engine/shell/RCommand.java

@@ -299,6 +299,7 @@ public class RCommand {
         return RRuntime.asString(RRuntime.asAbstractVector(RContext.getInstance().stateROptions.getValue("continue")));
     }
 
+    @SuppressWarnings("unused")
     private static void emitIO() throws IOException {
     }
 }

com.oracle.truffle.r.nodes.builtin/src/com/oracle/truffle/r/nodes/builtin/fastr/FastRInterop.java

@@ -61,6 +61,7 @@ public class FastRInterop {
             return callTarget.call();
         }
 
+        @SuppressWarnings("unused")
         private void emitIO() throws IOException {
         }
     }

com.oracle.truffle.r.test/src/com/oracle/truffle/r/test/generate/FastRSession.java

@@ -292,6 +292,7 @@ public final class FastRSession implements RSession {
         return "FastR";
     }
 
+    @SuppressWarnings("unused")
     static void emitIO() throws IOException {
     }
 }