Revert glyph warnings

[tacaswell]

This are producing many warnings on travis, reverting until we sort out why.

[efiring]

It stops the warnings; the Travis failure is unrelated.

[anntzer]

So the culprit is

matplotlib/lib/matplotlib/backends/backend_pdf.py

Line 938 in 63d446f

def get_char_width(charcode):

where we try to get the widths of all characters in the 0-255 range regardless of whether they exist in the font or not. The correct (and faster) approach is probably to just read the max_advance_width entry in the FT_Face structure (https://www.freetype.org/freetype2/docs/reference/ft2-base_interface.html#FT_FaceRec), but I have no problem with reverting this in the meantime.

Also, it would be nice to clean the warnings that occur during the test run and just make it a failure to have any warning emitted in the test suite (saying this as someone who already spammed the CI twice with repetitive warnings...).