Revert "Adding the path to gecode headers to build Scenario project"

This reverts commit 72ae979b510cf219dc2e78bce12ad06dcc0fcf11.
Revert "Adding gecode headers to the Scenario project"

This reverts commit ce1e5dfadfb7e53ec21ef454fc1652abf8fe727c.
Revert "Updating Scenario solver to use Gecode 4.2.0"

This reverts commit 4d001be7165bdc828ff37b31974e5af385d8f478.
Switching back on Gecode 3.7.3