assertions: Eliminate __return_from_bats_assertion
assertions: Eliminate __return_from_bats_assertion

A step towards #51. I was mistaken in #74 when I removed the scrubbing
of `BATS_CURRENT_STACK_TRACE[0]`. It turns out that when calling
`return_from_bats_assertion` directly, this value will need to be
removed.

This became apparent after removing `__return_from_bats_assertion` as a
step towards making `set -o functrace` conditional upon the immediate
caller per #51.

mbland/go-script-bash