borrowck: Inline `free_region_constraint_info()` to simplify the code
For reference, the helper function was introduced in 2a3969a3f7. But, the code becomes simpler if we remove the helper function, so let's remove it.
This takes us one step closer (this is commit 3 of 4) towards the end result illustrated in https://github.com/rust-lang/rust/pull/158623.
Jacob Pratt
86de60d933