Remove superfluous local variable

The collections have been checked for equal size, we then only need the
size of one of them to iterate. The other boundary pointer is never used
and only generate needless compiler warnings that pollutes the output.

tests/test_column_extract.cpp

@@ -149,7 +149,7 @@ BOOST_AUTO_TEST_CASE(DisjointColumn)
 
     for (VVI::iterator
         xb = correct_answer.begin(), xe = correct_answer.end(),
-        cb = columns       .begin(), ce = columns       .end();
+        cb = columns       .begin();
         xb != xe; ++xb, ++cb) {
 
         BOOST_CHECK_EQUAL_COLLECTIONS((*xb).begin(), (*xb).end(),