Fix ESLint issues (Errors) - no-unreachable, no-unused-vars, no-undef

[jazhen]

Fix all errors under the category no-unreachable, no-unused-vars, and no-undef.

List of issues (May not be exhaustive so double check):

• wtt_server/server/routes/treeadoptions/treeadoptions-router.js
  • 20:5 error Unreachable code no-unreachable
• wtt_server/server/routes/treeadoptions/treeadoptions.test.js
  • 208:15 error 'adoptedTree' is assigned a value but never used no-unused-vars
• wtt_server/server/routes/treeid/treeid-test.js
  • 2:8 error 'faker' is defined but never used no-unused-vars
• wtt_server/server/routes/treeid/treeid-validations.js
  • 7:7 error 'state' is not defined no-undef
• wtt_server/server/routes/treelikes/treelikes.test.js
  • 205:15 error 'likedTree' is assigned a value but never used no-unused-vars

Closes #109

[zoobot]

Nice thanks!