RTF Input: Add option to ignore WMF images

RTF Input: Add option to ignore WMF images iinstead of replacing them
with a placeholder. Fixes #1213599 [RTF input conversion creates too large an image when failing to convert .wmf file](https://bugs.launchpad.net/calibre/+bug/1213599)
pep8
Switch to newdb

Remove the use_newdb_tweak and various bits of code used to support
dual use.