Delete tool_public_key and tool_private_key fields from the lti_tool table

Back in ancient history (2 years ago) we thought tools would not be able to mint their own public private pairs so we had LMS's mint them and provide them.

This was always a very bad idea (sending private keys around) and while the earliest implementations of LTI Advantage in the LMS's supported it - by the time the last LMS (i.e. D2L) implemented LTI Advantage - all the major LMS's insisted that tool produce a keyset URL and had no support for this feature.

So this is vestigial code and needs to be removed.  SAK-44105 hid them - but this fix will remove them completely.