[SPARK-3476] Remove outdated memory checks in Yarn

See description in [JIRA](https://issues.apache.org/jira/browse/SPARK-3476).

Author: Andrew Or <andrewor14@gmail.com>

Closes #2528 from andrewor14/yarn-memory-checks and squashes the following commits:

c5400cd [Andrew Or] Simplify checks
e30ffac [Andrew Or] Remove outdated memory checks