This reverts commit 7926045ff5.
`org-src-associate-babel-session' is still useful because:
1. It indicates the need to implement session association to babel
backend (although we still need to make it more clear for backend
authors in future).
2. It provides infrastructure for more granular control over session
association for third-party packages. For example, we might want
to disabled session association when it can cause unwanted side
effects, like starting comint process unconditionally (looking at
ob-R and other ess-based backends).