This reverts commit e64cd3ae.

Reason for revert: A never macOS 11 beta made it so that
native arm binaries need to be signed, and the Xcode beta used
to build this arm64 slice here didn't do ad-hoc signing by default
yet, see https://crbug.com/1125021

Original change's description:
> mac: Add an arm64 slice to ninja-mac.
> 
> To build the arm ninja binary, I downloaded a zip with the ninja 1.8.2
> code, and ran `./configure --bootstrap` on an arm mac, with Xcode 12 beta
> installed, followed by `strip ninja`. That gave me a working ninja 1.8.2
> arm binary. I then copied it to my intel mac and ran
> 
>     lipo -create -output ninja-mac ninja-mac ~/Downloads/ninja
> 
> to make a fat binary.
> 
> No intended behavior change on non-arm macs.
> 
> (This means the x86_64 slice, which is the previously existing binary,
> has a LC_VERSION_MIN_MACOSX version 10.6, while the arm64 slice has
> LC_BUILD_VERSION minos 11.0, according to `otool -arch ax86_64 -l ninja-mac`
> and `otool -arch arm64 -l ninja-mac`.)
> 
> Bug: 1103315,762354
> Change-Id: I2c4bea53a01d7a6135f9cd5554e3123dba960664
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2288417
> Auto-Submit: Nico Weber <thakis@chromium.org>
> Reviewed-by: Mark Mentovai <mark@chromium.org>
> Commit-Queue: Mark Mentovai <mark@chromium.org>
> Commit-Queue: Nico Weber <thakis@chromium.org>

TBR=thakis@chromium.org,dpranke@google.com,mark@chromium.org,infra-scoped@luci-project-accounts.iam.gserviceaccount.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 1103315
Bug: 762354
Change-Id: Icd968a2fecb9509722c43d9cb03db92e6e3829e4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2393775Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>