Revert "Skip failing part of test-json"

This reverts commit 79a1c5f8e8.

The issue was fixed upstream.

systemd.spec

@@ -275,10 +275,6 @@ They can be useful to test systemd internals.
 %prep
 %autosetup -n %{?commit:%{name}%{?stable:-stable}-%{commit}}%{!?commit:%{name}%{?stable:-stable}-%{github_version}} -p1 -Sgit
 
-# Disable a failing test with gcc-9. This looks like a real issue, but
-# this only occurs on "fringe" architectures and I don't know to fix this.
-sed -r -i 's/test_build\(\);/\/\/\0/' src/test/test-json.c
-
 %build
 %define ntpvendor %(source /etc/os-release; echo ${ID})
 %{!?ntpvendor: echo 'NTP vendor zone is not set!'; exit 1}