doc: Remove old changelog fragments

doc/changes/auxiliary/mr.597.1.md

-os/time: Make timespec argument const.

doc/changes/auxiliary/mr.597.2.md

-os/time: Add a Linux specific way to get the realtime clock (for RealSense).

doc/changes/auxiliary/mr.603.md

-math: Make sure that we do not drop and positions in poses when the other pose
-has a non-valid position.

doc/changes/auxiliary/mr.605.md

-aux/vk: `vk_create_device` now takes in a list of Vulkan device extensions.

doc/changes/auxiliary/mr.627.md

-Port everything to u_logging.

doc/changes/auxiliary/mr.635.md

-u/hand_tracking: Tweak finger curl model making it easier to grip ingame objects.

doc/changes/auxiliary/mr.659.md

-math: Add math_quat_validate_within_1_percent function.

doc/changes/auxiliary/mr.665.md

-u/sink: Add Bayer format converter.

doc/changes/auxiliary/mr.666.md

-u/distortion: Improve both Vive and Index distortion by fixing polynomial math.

doc/changes/auxiliary/mr.667.md

-u/distortion: Improve Index distortion and tidy code. While this touches the
-Vive distortion code all Vive headsets seems to have the center set to the same
-for each channel so doesn't help them. And Vive doesn't have the extra
-coefficient that the Index does so no help there either.

doc/changes/big/mr.583.md

-Adds a initial SteamVR driver state tracker and target that produces a SteamVR
-plugin that enables any Monado hardware driver to be used in SteamVR. This is
-the initial upstreaming of this code and has some limitations, like only having
-working input when emulating a Index controller.

doc/changes/compositor/mr.591.1.md

-comp: Add basic frame timing information to XRT_COMPOSITOR_LOG=trace.

doc/changes/compositor/mr.599.md

-main: Refactor how the compositor interacts with targets, the goal is to enable
-the compositor to render to destinations that isn't backed by a `VkSwapchain`.
-Introduce `comp_target` and remove `comp_window`, also refactor `vk_swapchain`
-to be a sub-class of `comp_target` named `comp_target_swapchain`, the window
-backends now sub class `comp_target_swapchain`.

doc/changes/compositor/mr.620.md

----
-- mr.620
-- mr.624
----
-Implement support for XR_KHR_composition_layer_equirect (equirect1).

doc/changes/compositor/mr.645.md

-comp: Improve thread safety. Resolve issues in mutlithreading CTS.

doc/changes/compositor/mr.671.md

-main: Lower priority on sRGB format. This works around a bug in the OpenXR CTS
-and mirrors better what at least on other OpenXR runtime does.

doc/changes/drivers/mr.564.md

-North Star: Fix memory leak in math code.

doc/changes/drivers/mr.597.md

-psvr: Rename some variables for better readability.

doc/changes/drivers/mr.600.2.md

-openhmd: Fix viewport calculation of rotated displays.

doc/changes/drivers/mr.604.md

-remote: Add support for simulated hand tracking, this is based on the curl model
-that is used by the Valve Index Controller.