Drop patch for compatibility with pyreadline 1.5

Closes gh-1307

Needs testing on Windows.

#1307 implies that we have other compatibility things, but it's not clear what they are. There's [this check](https://github.com/ipython/ipython/blob/master/IPython/core/interactiveshell.py#L1824) in init_readline(), and pyreadline does now have [set_startup_hook](https://github.com/pyreadline/pyreadline/blob/master/pyreadline/rlmain.py#L211), but it does something different from `set_pre_input_hook`, and I don't know when it was added.