Remove ZAP Config file environment variables

Remove support for the following 3 environment variables as they do not work properly or align with the functionality of DAST. These variables are designed as an interface to the zap python scripts, not to ZAP or DAST and therefore we do not want to support them.

VARIABLE	required	Description
DAST_ZAP_GENERATE_CONFIG	no	The file name of the generated sample ZAP config file for use with DAST_ZAP_CONFIG_FILE.
DAST_ZAP_CONFIG_FILE	no	Name of config file used to determine thresholds of vulnerability rules.
DAST_ZAP_CONFIG_URL	no	URL of config file used to determine thresholds of vulnerability rules.