Revert "siso: fix max_batch_total_size_bytes handling"

This reverts commit 417b62505e392067fa7452cbddbe4bd4a3848240.

Reason for revert:
https://g-issues.chromium.org/issues/426055183#comment162
> W0912 09:31:18.722907 3972119 retry.go:122] 21f8f4cd-53c3-49fc-9e2a-b1e91e8c1ef3 retry backoff=346.23306ms: rpc error: code = ResourceExhausted desc = gRPC message exceeds maximum size 4194304: 4196713


Original change's description:
> siso: fix max_batch_total_size_bytes handling
>
> According to the spec,
> https://github.com/bazelbuild/remote-apis/blob/2c3c141e0e4ca43f08abf7ef9046f15e059c3214/build/bazel/remote/execution/v2/remote_execution.proto#L2231
> > Maximum total size of blobs to be uploaded/downloaded using batch methods.
>
> Siso calculated the request size accurately before.
>
> Bug: 440815127
> Change-Id: Ib222e450389de7efc85b5c35319ae6d2165b1d8c
> Reviewed-on: https://chromium-review.googlesource.com/c/build/+/6937850
> Commit-Queue: Fumitoshi Ukai <ukai@google.com>
> Auto-Submit: Junji Watanabe <jwata@google.com>
> Reviewed-by: Fumitoshi Ukai <ukai@google.com>

Bug: 440815127, 426055183
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Change-Id: Ie9c774fd01a3f3f9f74b062dab34a34bebd95fa2
Reviewed-on: https://chromium-review.googlesource.com/c/build/+/6939151
Auto-Submit: Fumitoshi Ukai <ukai@google.com>
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>

• siso/reapi/cas.go[diff]