Revert of Add tools/mb to setup_links.py (patchset #1 id:1 of https://codereview.webrtc.org/1692543002/ )

Reason for revert:
Doesn't solve the problem and MB is actually hardcoded to use gyp_chromium when running in GYP mode, so it's better to clean this up for now.

Original issue's description:
> Add tools/mb to setup_links.py
>
> I believe this is needed in order to setup the iOS simulator bots
> using the ios recipe module. We'll also most likely use MB in the future
> when moving over from GYP to GN (since that's the sole purpose of the tool:
> to make such a migration easier).
>
> BUG=chromium:498746
> NOTRY=True
> TBR=phoglund@webrtc.org
>
> Committed: https://crrev.com/68da769ab9579cac98fe4da7fb5da6dbb3842216
> Cr-Commit-Position: refs/heads/master@{#11575}

TBR=phoglund@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=chromium:498746

Review URL: https://codereview.webrtc.org/1691723003

Cr-Commit-Position: refs/heads/master@{#11577}

setup_links.py

@@ -69,7 +69,6 @@ DIRECTORIES = [
   'tools/generate_stubs',
   'tools/gn',
   'tools/gyp',
-  'tools/mb',
   'tools/memory',
   'tools/protoc_wrapper',
   'tools/python',