This commit removes declarations and mentions to the built-in `bold`
and `italic` faces.
While they could be a user-friendly way of customising how tokens
are emphasised in Markdown documents (similarly to the
`$LESS_TERMCAP_*` environment variables for `man` pagers), most other
markup languages do not have the concept of "strong" and "emphasis"
but refer directly to the font style/weight.
The faces were also not even set by default to highlight as their
names implied, so having markup language support scripts directly
use the +b and +i face attributes is more consistent.
