Remove obsolete Drupal core checks

Problem/Motivation

There are checks against version 10.3 of Drupal core that were added when the 3.1.x branch supported various versions of core. Since the 3.4.x branch requires 10.3 or higher, these checks are no longer needed.

Proposed resolution

Remove the code for versions prior to 10.3 and use only the 10.3 compatible code.