Revert "Set use_siso=true by default in .gn"

This reverts commit 0d98c5f245a09fad152da51f9a6846393ceae056.

Reason for revert: The CL disabled remote execution on non-corp
builds unexpectedly.

Bug: chromium:412968361
Original change's description:
> Set use_siso=true by default in .gn
>
> This CL enables use_siso=true by default.
> Googlers builds will switch to Siso, or get suggestion to run
> `gn clean` to switch.
> External developers will keep using Ninja for now. But, they will
> also use Siso soon.
>
> Bug: chromium:412968361
> Change-Id: I064ea6147135c76a1d1eb7b8ee733b5d9b04f1dd
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/393680
> Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
> Reviewed-by: Christoffer Dewerin <jansson@webrtc.org>
> Commit-Queue: Christoffer Dewerin <jansson@webrtc.org>
> Auto-Submit: Junji Watanabe <jwata@google.com>
> Cr-Commit-Position: refs/heads/main@{#44765}

Bug: chromium:412968361
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Change-Id: I5eb173a28e8418c27372c2b0e14230ac3d3a5e49
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/393880
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Christoffer Dewerin <jansson@webrtc.org>
Bot-Commit: rubber-stamper@appspot.gserviceaccount.com <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Junji Watanabe <jwata@google.com>
Cr-Commit-Position: refs/heads/main@{#44781}
diff --git a/.gn b/.gn
index 1b112b3..fbd2c38 100644
--- a/.gn
+++ b/.gn
@@ -91,7 +91,4 @@
 
   # See context on https://issues.chromium.org/issues/415076048.
   enable_js_protobuf = false
-
-  # Use Siso instead of Ninja.
-  use_siso = true
 }