pkgconfig: remove non-existent requirement on 'sandbox-2.0'

$ pkg-config --cflags libvirt-sandbox-1.0
Package sandbox-2.0 was not found in the pkg-config search path.
Perhaps you should add the directory containing `sandbox-2.0.pc'
to the PKG_CONFIG_PATH environment variable
Package 'sandbox-2.0', required by 'libvirt-sandbox-1.0', not found

https://bugzilla.redhat.com/show_bug.cgi?id=1327908

libvirt-sandbox-1.0.pc.in

@@ -6,6 +6,5 @@ includedir=@includedir@
 Name: libvirt-sandbox
 Version: @VERSION@
 Description: libvirt sandbox library
-Requires: sandbox-2.0
 Libs: -L${libdir} -lvirt-sandbox-1.0 @LIBVIRT_GOBJECT_LIBS@
 Cflags: -I${includedir}/libvirt-sandbox-1.0 @LIBVIRT_GOBJECT_CFLAGS@