Remove buildbot Nougat Phone Tester from milo

This hasn't had any builds since 04/2018

Bug: 790146
Change-Id: I867b60e3dd9e4250fc739aa4ec13d97f32e65c23
Reviewed-on: https://chromium-review.googlesource.com/c/1475168
Commit-Queue: Brad Hall <bradhall@google.com>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632676}

infra/config/luci-milo.cfg

@@ -1792,12 +1792,6 @@ consoles {
     category: "Testers"
     short_name: "low"
   }
-  builders {
-    name: "buildbot/chromium.android.fyi/Nougat Phone Tester"
-    name: "buildbucket/luci.chromium.ci/Nougat Phone Tester"
-    category: "Testers"
-    short_name: "N"
-  }
   builders {
     name: "buildbot/chromium.android.fyi/Memory Infra Tester"
     name: "buildbucket/luci.chromium.ci/Memory Infra Tester"