V8: Remove deprecated builder from MB config

The builder on master side was removed in https://crrev.com/c/806035.

R=machenbach@chromium.org

Bug: 747960
Change-Id: I1a1b145f0eff4c8854583b37e5959c75c1260d76
Reviewed-on: https://chromium-review.googlesource.com/806455Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Sergiy Byelozyorov <sergiyb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521701}

tools/mb/mb_config.pyl

@@ -793,7 +793,6 @@
     },
 
     'tryserver.v8': {
-      'v8_android_chromium_gn_dbg': 'android_debug_bot',
       'v8_linux_blink_rel': 'release_trybot',
       'v8_linux_chromium_gn_rel': 'release_trybot',
     },