3101 - purge .traces/ dir from repo history

I'd been toying with this idea for some time now given how large the
repo had been growing. The final straw was noticing that people cloning
the repo were having to wait *5 minutes*! That's not good, particularly
for a project with 'tiny' in its description. After purging .traces/
clone time drops to 7 seconds in my tests.

Major issue: some commits refer to .traces/ but don't really change
anything there. That could get confusing :/

Minor issues:

a) I've linked inside commits on GitHub like a half-dozen times online
or over email. Those links are now liable to eventually break. (I seem
to recall GitHub keeps them around as long as they get used at least
once every 60 days, or something like that.)

b) Numbering of commits is messed up because some commits only had
changes to the .traces/ sub-directory.

1 files changed, 0 insertions, 4 deletions

diff --git a/sandbox/Readme.md b/sandbox/Readme.md
index 5a6d6336..f2cb2fd9 100644
--- a/sandbox/Readme.md
+++ b/sandbox/Readme.md
@@ -18,7 +18,3 @@ either side to run the sandbox.
 
 Known issues: you have to explicitly save inside your editor before hitting
 F4, unlike with 'mu edit'.
-
-One problem with a fork of the edit/ app is that many of the tests write to
-the same files under the .traces/ directory. I try to always commit the
-results of edit/ rather than the sandbox/ app, but I haven't been perfect.