Remove one of the duplicate Solidity source directories

[pirapira]

Before this PR the repo had two identical directories raiden_contracts/contracts and raiden_contracts/data/source. This PR removes raiden_contracts/contracts. In the future, each raiden_contracts/data_x.y.z/source can be compiled separately from each other.

This closes #740 .

[codecov]

Codecov Report

Merging #741 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #741   +/-   ##
=======================================
  Coverage   81.59%   81.59%           
=======================================
  Files          18       18           
  Lines        1114     1114           
  Branches      106      106           
=======================================
  Hits          909      909           
  Misses        176      176           
  Partials       29       29

Impacted Files	Coverage Δ
raiden_contracts/contract_source_manager.py	90.27% <100%> (ø)	⬆️

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