(Refactor) Remove str_eq

[Janther]

• What is it? (leave one option)

  • (Refactor) of existing code (no functionality change)

• What was the root cause of the problem originally / what feature was missing?

The only use of str_eq is for comparison with an empty string. So I removed this to clean unused code.

• How does this pull request solve it (in broad terms)? Requiring the byte length of a string to be greater than 0.

• Does it close any open issues? Addresses https://github.com/poanetwork/poa-popa/pull/84#issuecomment-374244836

• Quick checklist

  • [x] docs were updated where necessary OR they don't need to be updated
  • [x] tests were updated where necessary OR they don't need to be updated

[coveralls]

Pull Request Test Coverage Report for Build 229

• 6 of 7 (85.71%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 68.573%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
blockchain/contracts/ProofOfPhysicalAddress.sol	6	7	85.71%
<!--	Total:	6	7	85.71%	-->

Totals
Change from base Build 227:	0.1%
Covered Lines:	759
Relevant Lines:	1060

---

💛 - Coveralls