fonts: Reduce the public API surface (#39112)

This change marks all items that are not used by other crates as
`pub(crate)` instead of `pub`.

Additionally, I've removed all code that turned out to be unused. I
think most of it was used only by `layout-2013`.


Testing: The correctness of these changes is verified by the compiler,
so no tests are needed.

---------

Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>

components/fonts/shapers/mod.rs

@@ -9,7 +9,7 @@ use app_units::Au;
 use base::text::is_bidi_control;
 use euclid::default::Point2D;
 use fonts_traits::ByteIndex;
-pub use harfbuzz::{ShapedGlyphData, Shaper};
+pub(crate) use harfbuzz::Shaper;
 use log::debug;
 use num_traits::Zero as _;