Revert "Switch to use CIPD ninja v1.8.2"

This reverts commit c90a982106.

Reason for revert: 
https://logs.chromium.org/logs/chromium/buildbucket/cr-buildbucket/8811949917908303825/+/u/compile/stdout

Original change's description:
> Switch to use CIPD ninja v1.8.2
>
> https://crrev.com/c/3674981 installed CIPD ninja packages.
> This CL switches from the old ninja binaries to them.
>
> Note that this CL doesn't change ninja version. the old/CIPD ninja binaries are v1.8.2.
>
> The differences from the old binaries are
> - the CIPD packages don't use chromium's clang.
> - the CIPD package for Linux doesn't use chromium's sysroot.
> - the min macOS version is 10.13, which was 10.6.
> Please also see https://crsrc.org/i/3pp/ninja/install_bootstrap.sh
>
>
> On Windows, ninja.exe will be installed under .cipd_bin.
> But there are many places that assume that ninja.exe exists on depot_tools root. So this CL also copies the ninja.exe from .cipd_bin to root in cipd_bin_setup.bat.
>
> Bug: 931218
> Change-Id: Ib67eee5e9b6ad9b2937b789626970d9c85867dbd
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3674982
> Commit-Queue: Junji Watanabe <jwata@google.com>
> Reviewed-by: Philipp Wollermann <philwo@google.com>
> Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
> Reviewed-by: Nico Weber <thakis@chromium.org>
> Reviewed-by: Josip Sokcevic <sokcevic@google.com>

Bug: 931218
Change-Id: I08d61dad119a0d23ae9ec8b1ed787dd3915de697
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3686854
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Owners-Override: Takuto Ikuta <tikuta@chromium.org>

.gitignore

@@ -6,9 +6,6 @@
 /.cipd_client*
 /.versions
 
-# ignore ninja.exe copied from .cipd_bin.
-/ninja.exe
-
 # Ignore "disable auto update" sentinel file.
 .disable_auto_update