Get rid of the need for deployer.pub (#385)

The tests still work because we manually write the deployer's public key in `/root/.ssh/authorized_keys` on the target machines. In itself, however, the configuration that we push does not allow the deployer to push anything on the target machines.

Context: Fediversity/Fediversity#361 (comment)
Reviewed-on: Fediversity/Fediversity#385
Reviewed-by: kiara Grouwstra <kiara@procolix.eu>
Co-authored-by: Nicolas “Niols” Jeannerod <nicolas.jeannerod@moduscreate.com>
Co-committed-by: Nicolas “Niols” Jeannerod <nicolas.jeannerod@moduscreate.com>

deployment/check/cli/deployer.pub

@@ -1 +0,0 @@
-## This is a placeholder file. It will be overwritten by the test.

deployment/check/common/nixosTest.nix

@@ -119,7 +119,6 @@ in
       with subtest("Configure the deployer key"):
         deployer.succeed("""mkdir -p ~/.ssh && ssh-keygen -t rsa -N "" -f ~/.ssh/id_rsa""")
         deployer_key = deployer.succeed("cat ~/.ssh/id_rsa.pub").strip()
-        deployer.succeed(f"echo '{deployer_key}' > ${config.pathFromRoot}/deployer.pub")
         ${forConcat config.targetMachines (tm: ''
           ${tm}.succeed(f"mkdir -p /root/.ssh && echo '{deployer_key}' >> /root/.ssh/authorized_keys")
         '')}