MDL-46148 qtype_calculated: removed unused method.
MDL-46148 qtype_calculated: fix validation when importing.

In order to do this in a sane way, I cleaned up a lot of old mess,
inclduing:

1. Previously, qtype_calcuated used ->answeres when importing, and
->answer when saving the form. This was crazy, so I fixed it, and
stripped out the code that made the alternative variable name work.

2. Similarly, it could handle ->answer being either an array, such as
you would get form the HTML editor, or a simple string, which is what
you get form the form. I simplified that too.

3. Finally, I made import use a transaction around saving each
question, so we don't get half questions in the database when an error
occurs.
MDL-46148 qtype_calculatedsimple: fix notice

... when adding a dataset with a formula error.
MDL-46148 questions: Added missing allow_commit for transaction
MDL-46148 qtype_calculated: low-level defence against bad formulas

This catches things like:
 * Malicious equations coming from backup files.
 * Malicious equations in old questions in the database.