[BUG][Kernel] Remove the `MixedDataType` from Kernel data types

[vkorukanti]

Bug

Which Delta project/connector is this regarding?

• [ ] Spark
• [ ] Standalone
• [ ] Flink
• [x] Kernel
• [ ] Other (fill in here)

Describe the problem

Delta 3.0 version of Kernel APIs have data type (MixedDataType) that is not part of the spec, but was added to get around the issues with table schema encoded in JSON. More details refer here.

We should get rid of this class as it causes confusion on list of data type supported by the Kernel.

Willingness to contribute

The Delta Lake Community encourages bug fix contributions. Would you or another member of your organization be willing to contribute a fix for this bug to the Delta Lake code base?

• [ ] Yes. I can contribute a fix for this bug independently.
• [ ] Yes. I would be willing to contribute a fix for this bug with guidance from the Delta Lake community.
• [ ] No. I cannot contribute a bug fix at this time.

[allisonport-db]

Decision doc on how to deserialize delta schemas: https://docs.google.com/document/d/1UOlw498twX0OME1T4e66lV9daZz7OjY4YCw0WxK2Jzs/edit?usp=sharing