avoid leaving the old perldelta as a symlink

In a built tree, the name we copy perldelta.pod to is a symlink to
perldelta.pod, and if you blindly follow the git adds, you end up
adding a symlink to the commit.

unlink() the name we're about to overwrite before we overwrite it to
prevent that.
new perldelta for 5.19.10
bump to version 5.19.10 and fix the version number reference in op.c
prepare Module::CoreList for next release