[SPARK-18654][SQL] Remove unreachable patterns in makeRootConverter

## What changes were proposed in this pull request?

`makeRootConverter` is only called with a `StructType` value. By making this method less general we can remove pattern matches, which are never actually hit outside of the test suite.

## How was this patch tested?

The existing tests.

Author: Nathan Howell <nhowell@godaddy.com>

Closes #16084 from NathanHowell/SPARK-18654.