spec: Remove BuildRoot definition

None of currently supported distributions need that.
It was needed last for EL5 which is EOL now
Signed-off-by: NIgor Gnatenko <ignatenkobrain@fedoraproject.org>
Signed-off-by: NCole Robinson <crobinso@redhat.com>

libvirt.spec.in

@@ -255,7 +255,6 @@ Version: @VERSION@
 Release: 1%{?dist}%{?extra_release}
 License: LGPLv2+
 Group: Development/Libraries
-BuildRoot: %{_tmppath}/%{name}-%{version}-%{release}-root
 URL: https://libvirt.org/
 
 %if %(echo %{version} | grep -q "\.0$"; echo $?) == 1