Redundant custom rules checkbox in preferences

[enriozuni]

With the new preference page options added in this PR #417, the load custom rules checkbox in preferences seems to be a little redundant. It currently loads all the rules in the resources folder of the core plugin: the JCA, BC, Tink rulesets, and the rules from the project which is analyzed. I find this unnecessary as an option since there already are options for loading the rules from specific rulesets and also for loading other rules from the project that is to be analyzed. My suggestion would be to remove this option in the future.

If it is decided to keep this checkbox, a suggestion would be to change the name of it because it is a little ambiguous. A better name would be load all possible rules or something similar.