Remove erroneous comment

Window frame bound may be a non-literal expression. This is already
covered by existing test case
(`io.prestosql.tests.AbstractTestWindowQueries#testWindowFrames`).

presto-parser/src/main/antlr4/io/prestosql/sql/parser/SqlBase.g4

@@ -413,7 +413,7 @@ frameBound
     : UNBOUNDED boundType=PRECEDING                 #unboundedFrame
     | UNBOUNDED boundType=FOLLOWING                 #unboundedFrame
     | CURRENT ROW                                   #currentRowBound
-    | expression boundType=(PRECEDING | FOLLOWING)  #boundedFrame // expression should be unsignedLiteral
+    | expression boundType=(PRECEDING | FOLLOWING)  #boundedFrame
     ;
 
 

presto-parser/src/test/java/io/prestosql/sql/parser/TestStatementBuilder.java

@@ -92,6 +92,7 @@ public void testStatementBuilder()
                 ", rank() over (partition by depname order by salary desc)\n" +
                 ", sum(salary) over (order by salary rows unbounded preceding)\n" +
                 ", sum(salary) over (partition by depname order by salary rows between current row and 3 following)\n" +
+                ", sum(salary) over (partition by depname order by salary rows between current row and empno following)\n" +
                 ", sum(salary) over (partition by depname range unbounded preceding)\n" +
                 ", sum(salary) over (rows between 2 preceding and unbounded following)\n" +
                 "from emp");