Remove special case for macOS in httpbin installation script

This is not necessary any longer since the changes of 1b76ff4887
(Disable wxWebRequest tests on CI platforms without Python 3,
2021-05-16) as we require Python 3 for httpbin anyhow.

build/tools/httpbin.sh

@@ -35,12 +35,6 @@ httpbin_launch() {
             blinker==1.4 brotlipy==0.7.0 cffi==1.14.5 click==7.0 decorator==4.4.2
             itsdangerous==1.1.0 pycparser==2.20 raven==6.10.0 werkzeug==0.16.1'
             ;;
-
-        macOS)
-            # We use Python 2 under macOS 10.11 which doesn't have Python 3,
-            # and decorator >= 5 is incompatible with it too.
-            pip_explicit_deps='decorator==4.4.2'
-            ;;
     esac
 
     # Ensure that we have at least some version of pip.