Revert "Added std::forward calls to rev version of reduce_sum (Issue #1815)"
Revert "Added std::forward calls to rev version of reduce_sum (Issue #1815)"

This reverts commit 7f767600f6f2d6e52e9f159f03a5303f5d9d2ac8.

stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math