ob-tangle.el: A small fix and some refactoring

ob-tangle.el: A small fix and some refactoring

* ob-tangle.el (org-babel-tangle): Remove unused attempt of
prompting the user of the tangle file name since :tangle is
always set.  Don't prompt for a tangle file name when called
with two universal prefix arg outside of a src block.
Use `org-babel-tangle-single-block'.
(org-babel-tangle-single-block): New function.
(org-babel-tangle-collect-blocks): Use the new function.

Thanks to Rick Frankel who provided a patch for this fix.
The patch fixes this issue (quoting Rick's email):

"When attempting to tangle a single block, `org-babel-tangle'
would use `narrow-to-region', causing any header arguments not
on the "#+BEGIN_SRC" line to be excluded from the tangled file."