Removes `plist-get-sexp`

This function was not working as advertised.
Then `funcall` was evaluated too early and all the benefits of late evaluation for autoloads was lost.
Furthermore, this function was not really needed in the first place:

```
(use-package foo
  :config my-foo-function)
```

can easily be replaced by the following:

```
(use-package foo
  :config (my-foo-function))
```

Fixes the issue raised in #54.