Remove redundant`is_numeric` and other checks for DataType

[alamb]

arrow-rs has some functions to check data type, and the logic is also in datafusion, for example, https://docs.rs/arrow/7.0.0/arrow/datatypes/enum.DataType.html#method.is_numeric

We should reduce the redundancy in DataFusion to make the codebase clear.

Originally posted by @liukun4515 in #1606 (comment)

[alamb]

This is a good one -- just delete some code from DataFusion 👍

[HaoYang670]

is_numeric in arrow-rs seems not to be same as in arrow-datafusion because DataType::Decimal is not included.

[liukun4515]

is_numeric in arrow-rs seems not to be same as in arrow-datafusion because DataType::Decimal is not included.

@HaoYang670

In the PG or Spark, the decimal is included in the numeric.
Because the decimal is in development, some codes may be not consistent.

[liukun4515]

Now we can focus on other issues, and deal with them after other stabilization.

[HaoYang670]

At least, should rename this function, so that developers would not misuse this and DataType::is_numeric.