Google submissions do allow underscores

[yanokwa]

Closes #353 and reverts #299 because ODK Collect v1.23.3 allows submissions with underscores as shown here:

For further proof, getodk/collect@3cca371 shows that the code in Collect that does this check was removed.

In the future, we really should test these PRs end to end before merging.

[lognaturel]

To do a little post mortem here:

• in Feb 2018, a user posted on the forum with a screenshot from Collect showing an error message about underscores in column headers with Google Sheets
• some time before Mar 2018, Google Sheets started supporting underscores in column headers
• in Mar 2018, @yanokwa filed Add warnings to or_other users about Google Sheets compatibility #270 to warn about underscores in column headers
• also in Mar 2018, Support repeats in Google Sheets getodk/collect#1851 was merged. This PR was to add repeat support in the Google Sheets integration but it also snuck in getodk/collect@3cca371 which removes the error message the user had posted about
• in May 2019, @lincmba submitted Underscores in name warning #299 to warn about underscores in field names

This is a tough one because integrations are involved. We obviously we have no control over Google Sheets and the folks who work on Collect and on pyxform are not all the same.

Some things we can improve:

• file issues for problems found while solving another issue. If @grzesiek2010 or one of the reviewers had filed an issue for getodk/collect@3cca371, we might have better remembered that the behavior changed.
• do a little more root cause analysis before filing an issue like Add warnings to or_other users about Google Sheets compatibility #270. Maybe @yanokwa could have noticed then that the limitation was no longer valid.

At the end of the day, there are a lot of moving pieces here so not sure we could have done a whole lot better but I think it's helpful to think about what went wrong.