[ttml2] font*-p features

I'm having second thoughts about adding the font*-p features in
https://github.com/w3c/ttml2/pull/664/commits/640f28ede06c0175e5d260af0790fef913e643ac
.

The reason we added these is because we explicitly added 'p' as one of the
elements to which these font* properties applied. Now, why do they apply?
Only for the purpose of determining the default line height for a paragraph
(used to resolve 'normal'), since one has to resolve the font properties to
a specific font resource (in order to obtain the ascent, descent, and line
gap (separation) data from the font).

But the thing is: all TTML1 implementations had to use this algorithm
already, so effectively already applied these properties to 'p'. So in
reality, we aren't adding new functionality here (that might warrant having
new feature designators), but only rectifying a missing behavior
specification (in TTML1) that was already implemented in reality (by both
old implementations, like DFXPVW, and new implementations, like IMSCJS,
which maps to CSS (which already applies these font properties to P in this
fashion).

I recommend we remove the new font*-p features and instead add notes to
each of the properties that explains that the addition of 'p' to this list
of applied elements serves only to document existing behavior consistent
with TTML1 implementations.

However, if someone knows of a TTML1 (or IMSC) implementation that does not
use these semantics for obtaining the default line height of a paragraph,
then we will need to discuss this further.