Remove findfont module-level function in kiva.fonttools.font_manager

[kitchoi]

There are no evidence that the module-level kiva.fonttools.font_manager.findfont being used downstream nor within kiva's code base apart from tests.

e.g. Projects typically use the findfont method on the Font object, like this:

>> from kiva.fonttools import font
>>> f = font.Font()
>>> f.findfont()

xref: #488 (comment)