refactor: remove --hot-only in favor of --hot=only

[snitin315]

• This is a bugfix
• This is a feature
• This is a code refactor
• This is a test update
• This is a docs update
• This is a metadata update

For Bugs and Features; did you add new tests?

No, need a new webpack-cli release to enable tests

Motivation / Use-Case

Remove --hot-only as now cli allows --hot=only - webpack/webpack-cli#2444

Breaking Changes

--hot-only option was removed.

Additional Info

None

[codecov]

Codecov Report

Merging #3093 (5c5e400) into master (728effc) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3093   +/-   ##
=======================================
  Coverage   92.30%   92.30%           
=======================================
  Files          37       37           
  Lines        1261     1261           
  Branches      328      328           
=======================================
  Hits         1164     1164           
  Misses         92       92           
  Partials        5        5           

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