Revert "MINOR: istbuf: add b_fromist() to make a buffer from an ist"

This reverts commit 9e46496d45ff06317ae8f4f785e6117e5b786f6f. It was
wrong and is not reliable, depending on the compiler's version and
optimization, as the struct is assigned inside a statement, thus on
its own stack. It's not needed anymore now so let's remove this.
1 file changed