[SPARK-18917][SQL] Remove schema check in appending data

## What changes were proposed in this pull request?
In append mode, we check whether the schema of the write is compatible with the schema of the existing data. It can be a significant performance issue in cloud environment to find the existing schema for files. This patch removes the check.

Note that for catalog tables, we always do the check, as discussed in https://github.com/apache/spark/pull/16339#discussion_r96208357

## How was this patch tested?
N/A

Closes #16339.

Author: Reynold Xin <rxin@databricks.com>

Closes #16622 from rxin/SPARK-18917.