Revert "Modify chebfun.sum() to work for array-valued inputs."
Revert "Modify chebfun.sum() to work for array-valued inputs."

This reverts commit b39180e06acee36935ef21c0d09b3c942db2fbd7.

b39180e0 has been made obsolete by 9efc5160, 085d0dba, and 29cd9196.

ilovejs/chebfun,alshedivat/chebfun,asgeirbirkis/chebfun