remove platform_font_manager()

We don't need this name munging since the
native/non-native font bots split.

Change-Id: I0e64feb08441ece8e0e4be0a70b812220aa8385a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/199300
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>