cffi: Drop unnecessary libm substitutes

pkgs/development/python-modules/cffi/default.nix

@@ -15,18 +15,7 @@ if isPyPy then null else buildPythonPackage rec {
 
   propagatedBuildInputs = [ pycparser ];
 
-  # On Darwin, the cffi tests want to hit libm a lot, and look for it in a global
-  # impure search path. It's obnoxious how much repetition there is, and how difficult
-  # it is to get it to search somewhere else (since we do actually have a libm symlink in libSystem)
   prePatch = lib.optionalString stdenv.isDarwin ''
-    substituteInPlace testing/cffi0/test_parsing.py \
-      --replace 'lib_m = "m"' 'lib_m = "System"' \
-      --replace '"libm" in name' '"libSystem" in name'
-    substituteInPlace testing/cffi0/test_unicode_literals.py --replace 'lib_m = "m"' 'lib_m = "System"'
-    substituteInPlace testing/cffi0/test_zdistutils.py --replace 'self.lib_m = "m"' 'self.lib_m = "System"'
-    substituteInPlace testing/cffi1/test_recompiler.py --replace 'lib_m = "m"' 'lib_m = "System"'
-    substituteInPlace testing/cffi0/test_function.py --replace "lib_m = 'm'" "lib_m = 'System'"
-    substituteInPlace testing/cffi0/test_verify.py --replace "lib_m = ['m']" "lib_m = ['System']"
   '';
 
   # The tests use -Werror but with python3.6 clang detects some unreachable code.