control/controlhttp: reduce some log spam on context cancel

Change-Id: I3ac00ddb29c16e9791ab2be19f454dabd721e4c3
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f