Remove minimal and no_customization build flavors

requirement_files/requirements_minimal.txt

@@ -1,2 +0,0 @@
--r pyobjc.txt
--r xattr.txt

requirement_files/requirements_opinionated.txt

@@ -1,23 +0,0 @@
--r arrow.txt
--r asn1crypto.txt
--r aspy.yaml.txt
--r atomicwrites.txt
--r black.txt
--r boto.txt
--r docklib.txt
--r entrypoints.txt
--r flake8-bugbear.txt
--r funcsigs.txt
--r isort.txt
--r packaging.txt
--r pre-commit.txt
--r pyobjc.txt
--r pytest.txt
--r pytest-docker.txt
--r python-dotenv.txt
--r requests.txt
--r rsa.txt
--r slacker.txt
--r Sphinx.txt
--r tokenize-rt.txt
--r xattr.txt