internal/postgres: remove the exclusive lock in upsertPath

As described in golang/go#75959, the lock in upsertPath causes a
significant amount of contention during module inserts. Meanwhile, the
upsertPaths method does not acquire a lock, and does not seem to cause
deadlocks.

We still don't understand the deadlocks avoided by this lock, but it
seems that a low level of failed transactions due to deadlock may be
preferable to the contention we're seeing currently. Let's experiment
with removing the lock.

For golang/go#75959

Change-Id: I2d577b426724010e96ffc8129ae469825fdcb938
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/718720
kokoro-CI: kokoro <[email protected]>
Reviewed-by: Ethan Lee <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Jonathan Amsterdam <[email protected]>

Author: findleyr

internal/postgres/path.go

@@ -84,9 +84,6 @@ func upsertPath(ctx context.Context, tx *database.DB, path string) (id int, err
 	// fine.
 	defer derrors.WrapStack(&err, "upsertPath(%q)", path)
 
-	if _, err := tx.Exec(ctx, `LOCK TABLE paths IN EXCLUSIVE MODE`); err != nil {
-		return 0, err
-	}
 	err = tx.QueryRow(ctx,
 		`INSERT INTO paths (path) VALUES ($1) ON CONFLICT DO NOTHING RETURNING id`,
 		path).Scan(&id)