Remove utils functions existing in codemirror.

[Carreau]

These two utils functions seem to exist in codemirror

I guess they are more reliable.

[takluyver]

Because the line numbers on master are liable to change: the functions in our code are {to,from}_absolute_cursor_position, and in CodeMirror, posFromIndex and indexFromPos. +1 to removing ours if we can.

[minrk]

👍 to removing our own implementations if they are redundant.