Remove EOL'd Ubuntu from targets.yaml (#5887)

See https://wiki.ubuntu.com/Releases.

Ubuntu 15.* repositories have been shut down for months now causing our tests
to always fail on these systems. While the tests on Ubuntu 12.04 still work, it
has been unsupported by Canonical for almost a year and I don't think we should
hamstring ourselves trying to continue to support it ourselves.

tests/letstest/targets.yaml

@@ -1,16 +1,6 @@
 targets:
   #-----------------------------------------------------------------------------
   #Ubuntu
-  - ami: ami-26d5af4c
-    name: ubuntu15.10
-    type: ubuntu
-    virt: hvm
-    user: ubuntu
-  - ami: ami-d92e6bb3
-    name: ubuntu15.04LTS
-    type: ubuntu
-    virt: hvm
-    user: ubuntu
   - ami: ami-7b89cc11
     name: ubuntu14.04LTS
     type: ubuntu
@@ -21,11 +11,6 @@ targets:
     type: ubuntu
     virt: pv
     user: ubuntu
-  - ami: ami-0611546c
-    name: ubuntu12.04LTS
-    type: ubuntu
-    virt: hvm
-    user: ubuntu
   #-----------------------------------------------------------------------------
   # Debian
   - ami: ami-116d857a