Fix stdout being plugged into codemod-ed file

[josieesh]

Summary

• When running a codemod on a file, the output success message of black ended up embedded in the file itself. See image below:
  

• Tried to set the -q flag here to quiet the output but that ended up completely erasing the test file

• Removing the line of code in libcst/codemod/_cli.py did the trick. The file still gets reformatted and the black output message still ends up in the terminal. See image below:
  

Test Plan

• Tested a test file z.py with various existing codemod commands
• Tested with --no-format flag to ensure that when it's on, black is not called at all

[jimmylai]

Nice catch!
Set stderr=subprocess.STDOUT makes stderr output been captured as updated code.

We didn't run into this issue internally because we adds -q parameter to silence stderr when calling Black.

[codecov-commenter]

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