Remove unnecessary black exclude config

It seems to have been copy-pasted from the Black docs.

pyproject.toml

@@ -7,25 +7,6 @@ max-line-length=120
 [tool.black]
 line-length = 120
 target-version = ['py39']
-exclude = '''
-
-(
-  /(
-      \.eggs         # exclude a few common directories in the
-    | \.git          # root of the project
-    | \.hg
-    | \.mypy_cache
-    | \.tox
-    | \.venv
-    | _build
-    | buck-out
-    | build
-    | dist
-  )/
-  | foo.py           # also separately exclude a file named foo.py in
-                     # the root of the project
-)
-'''
 
 [tool.isort]
 line_length = 120