move to using sqlx master rather than local fork of it

[adobeDan]

Description
Now that launchbadge/sqlx#983 (support for modern tokio) has been merged to SQLX master, we should change our build to pull from SQLX again rather than our (out-of-date) fork of the source of that PR.

Let's wait a day or two to see if they do the 0.5beta release they are talking about doing, in which case we can just use that tag as the source. Otherwise we can grab a stable commit that has these changes.