elpa: Use `package-delete' to remove packages.

Simply removing the package directory is no longer enough (and why use
`dired' for it?).  This will only set the package status to `deleted'
but package.el will still think the package is installed and refuse a
re-install.  Calling `package-delete' will ensure the right thing is
done.

* methods/el-get-elpa.el (el-get-elpa-post-remove): Call
  `package-delete' to remove packages.