text: Title: Remove bytes support from `robot.utils.normalize` function
  Body: Our `normalize` utility is used for normalizing strings, but it also supports bytes. The reason for the bytes support is that in Python 2 `str` actually was bytes and we needed to handle that in addition to `unicode`. In Python 3 strings and bytes are separated properly, and this kind of string manipulation doesn't make sense with bytes. The bytes support makes the code a bit more complicated and removing it will also make the code a tiny bit faster.