Build: fix a rule in the opam file (the remove script is in etc/lutin, not etc/Lutin)

2 jobs for 2.70.4 in 8 minutes and 47 seconds (queued for 1 second)
latest
Status Name Job ID Coverage
  Build
passed build #92453

00:04:43

 
  Test
passed test_opam_test #92454

00:04:03