Remove l3backend. It's now integrated in l3kernel. (#514)

- https://www.mail-archive.com/ctan-ann@ctan.org/msg13747.html
- Resolves #513

Author: benz0li

tools/pkgs-custom.txt

@@ -46,7 +46,6 @@ knuth-lib
 kvdefinekeys
 kvoptions
 kvsetkeys
-l3backend
 l3kernel
 l3packages
 latex