Remove support for set mode from dcos dns

[urbanserj]

JIRA: https://jira.mesosphere.com/browse/DCOS_OSS-5641

[codecov]

Codecov Report

Merging #188 into master will increase coverage by 0.02%.
The diff coverage is 14.7%.

@@            Coverage Diff             @@
##           master     #188      +/-   ##
==========================================
+ Coverage   66.45%   66.47%   +0.02%     
==========================================
  Files          55       55              
  Lines        3756     3726      -30     
==========================================
- Hits         2496     2477      -19     
+ Misses       1260     1249      -11

Flag	Coverage Δ
#ct	46.61% <2.94%> (+0.31%)	⬆️
#ct2	46.61% <2.94%> (+0.31%)	⬆️
#eunit	24.39% <11.76%> (-0.37%)	⬇️

Impacted Files	Coverage Δ
apps/dcos_dns/src/dcos_dns_config.erl	88.57% <ø> (-0.32%)	⬇️
apps/dcos_dns/src/dcos_dns_listener.erl	35.71% <100%> (+2.38%)	⬆️
apps/dcos_dns/src/dcos_dns_mesos.erl	39.84% <12.12%> (-4.46%)	⬇️
apps/dcos_l4lb/src/dcos_l4lb_mgr.erl	80.19% <0%> (ø)	⬆️

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