Drop ShapePlanCache

The ShapePlanCache was added to improve performance when shaping the same
strings over and over. However, it never had the ability to be trimmed
and when it was moved to FontSystem, this created a permanently growing
allocation. It is recommended to instead use the shape-run-cache feature
which supports trimming if it is desired to have higher performance for
repeated shaping, at the cost of manually specifying when to trim.

src/lib.rs

@@ -132,9 +132,6 @@ mod line_ending;
 pub use self::shape::*;
 mod shape;
 
-use self::shape_plan_cache::*;
-mod shape_plan_cache;
-
 pub use self::shape_run_cache::*;
 mod shape_run_cache;