Remove invHTstd and test.

These were crucial in getting the second derivatives right for invHT, as they
let me test with geval. We don't use them in practice though, so I'm removing
them from the working tree. They're still in the git history, of course.

Revert "Fix comment in invHTstd."

This reverts commit 5256a6490262b9312652f5b09a80dffe1dac195e.

Revert "Add invHTstd and test."

This reverts commit 48fec36091f96580875158593ca502e8b7829f63.