Remove redundant sentence in comment

Change-Id: Ia9ab4ce31541162cfd156175562bd6b665f978b9

include/ceres/problem.h

@@ -413,12 +413,11 @@ class CERES_EXPORT Problem {
     // The set of residual blocks to evaluate. This vector determines
     // the order in which the residuals occur, and how the rows of the
     // jacobian are ordered. If residual_blocks is empty, then it is
-    // assumed to be equal to the vector containing all the residual
-    // blocks. If this vector is empty, then it is assumed to be equal
-    // to a vector containing ALL the residual blocks. Generally
-    // speaking the residual blocks will occur in the order in which
-    // they were added to the problem. But, this may change if the
-    // user removes any residual blocks from the problem.
+    // assumed to be equal to the vector containing ALL the residual
+    // blocks. Generally speaking the residual blocks will occur in
+    // the order in which they were added to the problem. But, this
+    // may change if the user removes any residual blocks from the
+    // problem.
     vector<ResidualBlockId> residual_blocks;
 
     // Even though the residual blocks in the problem may contain loss