ox: Remove `org-export-execute-babel-code'

ox: Remove `org-export-execute-babel-code'

* lisp/ox.el (org-export-execute-babel-code): Remove function.
(org-export-as): Replace previous function with
`org-babel-exp-process-buffer'.
* lisp/ob-exp.el (org-babel-exp-process-buffer): Change signature.
  Copy buffer before evaluating code.

* testing/lisp/test-ob-exp.el (ob-exp/exports-inline-code):
(ob-exp/exports-inline-code-double-eval):
(ob-exp/exports-inline-code-eval-code-once):
(ob-exp/exports-inline-code-double-eval-exports-both):
(ob-export/export-with-results-before-block):
(ob-export/export-src-block-with-switches):
(ob-export/export-src-block-with-flags):
(ob-export/export-and-indentation):
(ob-export/export-under-commented-headline):
(ob-export/reference-in-post-header):
(ob-export/babel-evaluate):
(org-test-with-expanded-babel-code):
* testing/lisp/test-ob-lob.el (test-ob-lob/export-lob-lines):
(test-ob-lob/do-not-eval-lob-lines-in-example-blocks-on-export): Apply
change.

This patch merges `org-export-execute-babel-code' within
`org-babel-exp-process-buffer', removing one level of indirection.  Now,
all the Babel evaluation is on the Babel side.