Drop mz for util.promisify

[malept]

• I have read the contribution documentation for this project.
• I agree to follow the code of conduct that this project follows, as appropriate.
• The changes are appropriately documented (if applicable).
• The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

Summarize your changes:

With the Node 8 changes, mz is no longer necessary. One less dependency!

[codecov]

Codecov Report

Merging #983 into v14-develop will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           v14-develop   #983   +/-   ##
==========================================
  Coverage          100%   100%           
==========================================
  Files               13     13           
  Lines              674    674           
==========================================
  Hits               674    674

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