Remove `xkcd.png`, which is disused.

Fixes #368.

Change-Id: Id51edac4cdf29f10d839bdc26330b8d4f5786499
Reviewed-on: https://code-review.googlesource.com/c/re2/+/59970
Reviewed-by: Jeff Bailey <jeffbailey@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>
2 files changed