Remove black excludes config

[dstansby]

black automatically ignores files already specified in .gitignore, so remove files that are duplicated across .gitignore and the black config.

The remaining entries were

exclude = '''
(
  /(
    | _build
    | buck-out
    | examples
  )/
)
'''

and I these should either be in .gitignore (_build), not be ignored by black (examples) or I don't know what they are (buck-out), so I've just removed the exclude config completely.