ob-eval: make org-babel--shell-command-on-region internal and simplify
* lisp/ob-eval.el (org-babel-eval): Use simplified version of
`org-babel--shell-command-on-region´, we are the only caller of this
function.
* lisp/ob-eval.el (org-babel--shell-command-on-region): Replace
`org-babel-shell-command-on-region´ with a much more simplified
internal version, remove superfluous DOCSTRING and interactive
clause, strip out all conditionals which were never used. Prevent
deletion of temporary input file to aid debugging when the symbol
`org-babel--debug-input´ is bound and has non-nil value.