Remove redundant numpy dependency (#17594)

Missed removing ``numpy`` from `setup.cfg` in #17575. It was only added in setup.cfg in https://github.com/apache/airflow/pull/15209/files#diff-380c6a8ebbbce17d55d50ef17d3cf906

numpy already has `python_requires` metadata: https://github.com/numpy/numpy/blob/v1.20.3/setup.py#L473

so we don't need to set `numpy<1.20;python_version<"3.7"`

setup.cfg

@@ -121,9 +121,6 @@ install_requires =
     markdown>=2.5.2, <4.0
     markupsafe>=1.1.1, <2.0
     marshmallow-oneofschema>=2.0.1
-    # Numpy stopped releasing 3.6 binaries for 1.20.* series.
-    numpy<1.20;python_version<"3.7"
-    numpy;python_version>="3.7"
     # Required by vendored-in connexion
     openapi-spec-validator>=0.2.4
     pendulum~=2.0