[SPARK-21789][PYTHON] Remove obsolete codes for parsing abstract schema strings

## What changes were proposed in this pull request?

This PR proposes to remove private functions that look not used in the main codes, `_split_schema_abstract`, `_parse_field_abstract`, `_parse_schema_abstract` and `_infer_schema_type`.

## How was this patch tested?

Existing tests.

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #18647 from HyukjinKwon/remove-abstract.