generator: remove the default sanitizer feature

Now that we have an Opt.sample_default attribute, it's not very clear
we have a good use case for the sanitizer feature. Any default that we'd
want to sanitize in this way could probably just use sample_default
instead.

blueprint: oslo-config-generator
Change-Id: Ifd02bb66a2033afdaa5812b15941d2d485165863
Merge "generator: remove the default sanitizer feature"