ssh/tailssh: remove unused public key support

When we first made Tailscale SSH, we assumed people would want public
key support soon after. Turns out that hasn't been the case; people
love the Tailscale identity authentication and check mode.

In light of CVE-2024-45337, just remove all our public key code to not
distract people, and to make the code smaller. We can always get it
back from git if needed.

Updates tailscale/corp#25131
Updates golang/go#70779

Co-authored-by: Percy Wegmann <[email protected]>
Change-Id: I87a6e79c2215158766a81942227a18b247333c22
Signed-off-by: Brad Fitzpatrick <[email protected]>

Author: bradfitz

Makefile

@@ -116,7 +116,6 @@ sshintegrationtest: ## Run the SSH integration tests in various Docker container
 	GOOS=linux GOARCH=amd64 ./tool/go build -o ssh/tailssh/testcontainers/tailscaled ./cmd/tailscaled && \
 	echo "Testing on ubuntu:focal" && docker build --build-arg="BASE=ubuntu:focal" -t ssh-ubuntu-focal ssh/tailssh/testcontainers && \
 	echo "Testing on ubuntu:jammy" && docker build --build-arg="BASE=ubuntu:jammy" -t ssh-ubuntu-jammy ssh/tailssh/testcontainers && \
-	echo "Testing on ubuntu:mantic" && docker build --build-arg="BASE=ubuntu:mantic" -t ssh-ubuntu-mantic ssh/tailssh/testcontainers && \
 	echo "Testing on ubuntu:noble" && docker build --build-arg="BASE=ubuntu:noble" -t ssh-ubuntu-noble ssh/tailssh/testcontainers && \
 	echo "Testing on alpine:latest" && docker build --build-arg="BASE=alpine:latest" -t ssh-alpine-latest ssh/tailssh/testcontainers