### What is this PR for?
We don't have to maintain PostgresqlInterpreter because JDBCInterpreter covers all functions of PostgresqlInterpreter. It reduces maintenance costs.
### What type of PR is it?
[Feature]
### Todos
* [x] - Remove files and lines related to PostgresqlInterpreter
### What is the Jira issue?
* https://issues.apache.org/jira/browse/ZEPPELIN-2003
### How should this be tested?
N/A
### Screenshots (if appropriate)
### Questions:
* Does the licenses files need update? No
* Is there breaking changes for older versions? Yes
* Does this needs documentation? Yes, but already documented in jdbc docs
Author: Jongyoul Lee <jongyoul@gmail.com>
Closes#1945 from jongyoul/ZEPPELIN-2003 and squashes the following commits:
5dc58f2 [Jongyoul Lee] Fixed doc
ec31f78 [Jongyoul Lee] Change docs to make a link to jdbc.md
bfd70ef [Jongyoul Lee] Removed Postgresql from docs and configurations
98a106e [Jongyoul Lee] Removed whole packages about Postgresql
9d90ccca69