Remove test contracts from codebase

[Endogen]

Everything that is related to tests should be in the tests folder and not in the normal contracting codebase. Currently we have the following two files in the codebase:

proxythis.py
https://github.com/xian-network/xian-contracting/blob/master/src/contracting/contracts/proxythis.py

thistest2.py
https://github.com/xian-network/xian-contracting/blob/master/src/contracting/contracts/thistest2.py

Both are only present for tests and should be removed. If we need them for tests, then define the content of the files in the test itself.