Clean up before every run to avoid working on already minified file

[buunguyen]

Fixes #538.

Approach taken: perform a cleanup before a every run. Alternative: cleanup at failure.

I chose the former because:

• It's more robust. There could be interruption other than failure.
• Upon failure, (I guess) the minified file might be useful for troubleshooting.

[codecov]

Codecov Report

Merging #557 into master will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #557      +/-   ##
==========================================
+ Coverage   82.89%   82.93%   +0.03%     
==========================================
  Files          40       40              
  Lines        2719     2719              
  Branches      952      952              
==========================================
+ Hits         2254     2255       +1     
+ Misses        281      280       -1     
  Partials      184      184

Impacted Files	Coverage Δ
packages/babel-plugin-minify-simplify/src/index.js	80.74% <0%> (+0.12%)	⬆️

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[boopathi]

Thanks.