Blender Git Loki

Cleanup: Remove `SMALL_TRI_RIGHT_VEC` icon

Since the recent change to context paths to use the arrow icon instead of the triangle (D13106),
the `SMALL_TRI_RIGHT_VEC`is no longer used. This patch removes the icon and all references.

- Replace `SMALL_TRI_RIGHT_VEC` with `RIGHTARROW` in Freestyle UI
- Remove references to `SMALL_TRI_RIGHT_VEC` and `ICON_SMALL_TRI_RIGHT_VEC`.

Fix for built-in add-ons has been done in rBAcc2f71bfe9b0/rBAa84028f8a89a.

This will be added to the list of breaking changes [[ https://wiki.blender.org/wiki/Reference/Release_Notes/3.0/Python_API#Breaking_Changes | in the Wiki ]].

Reviewed By: Severin

Differential Revision: https://developer.blender.org/D13130