Remove the outdated script references in two Quark Agent showcases

[haeter525]

In #685 , the following two Quark Agent showcases still use the outdated script, quarkScriptAgent.py.

1. Showcase: Detecting CWE-798 with Quark Agent
2. Showcase: Generating Quark Script with Quark Agent

Since the script has been merged into Quark Agent, we should remove it and update the showcases to use Quark Agent.