Drop fuchsia from clang roll trybot list.

The bot no longer exists by this name, and there's a fuchsia bot on
the regular CQ now anyway.

Bug: 
Change-Id: I4187b9de545ae36acccc38f719e9718136c1e353
Reviewed-on: https://chromium-review.googlesource.com/749622Reviewed-by: Hans Wennborg <hans@chromium.org>
Commit-Queue: Peter Collingbourne <pcc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513236}

docs/updating_clang.md

@@ -36,7 +36,7 @@
       -b ios-device && \
     git cl try -m tryserver.chromium.linux -b linux_chromium_chromeos_dbg_ng \
       -b linux_chromium_chromeos_asan_rel_ng -b linux_chromium_msan_rel_ng \
-      -b fuchsia -b linux_chromium_cfi_rel_ng &&
+      -b linux_chromium_cfi_rel_ng &&
         git cl try -m tryserver.blink -b linux_trusty_blink_rel
     ```