Don't force the use of a specific DB index

[orien]

We include a work around for a MySQL query optimiser defect
https://github.com/envato/double_entry/blob/master/lib/double_entry/balance_calculator.rb#L42-L48
https://github.com/envato/double_entry/blob/master/lib/double_entry/validation/line_check.rb#L55-L63

This MySQL defect has been patched (in 2010). Is it time to remove the work around?

[asellitt]

can we benchmark with and without?

[orien]

Reading @notahat's comments in the bug report. It doesn't sound deterministic.

Perhaps putting a flip around this is a way forward. Move forward with the change, while allowing a fast way to revert the behaviour if everything goes wrong.

[orien]

This is not an urgent fix. The index requested is still the appropriate index to use. We don't gain much from doing this work.