[SPARK-18663][SQL] Simplify CountMinSketch aggregate implementation

## What changes were proposed in this pull request?
SPARK-18429 introduced count-min sketch aggregate function for SQL, but the implementation and testing is more complicated than needed. This simplifies the test cases and removes support for data types that don't have clear equality semantics:

1. Removed support for floating point and decimal types.

2. Removed the heavy randomized tests. The underlying CountMinSketch implementation already had pretty good test coverage through randomized tests, and the SPARK-18429 implementation is just to add an aggregate function wrapper around CountMinSketch. There is no need for randomized tests at three different levels of the implementations.

## How was this patch tested?
A lot of the change is to simplify test cases.

Author: Reynold Xin <rxin@databricks.com>

Closes #16093 from rxin/SPARK-18663.