[layout-unit] Remove unused code in `ApplySpacingImpl`

The `total_space`, a local variable of `ApplySpacingImpl`,
is adjusted at the end of the function, but the adjusted
value isn't used. This patch removes the adjustment.

This patch should have no behavior changes.

Bug: 351092748
Change-Id: I86335c03221cbe3dfbd54f21b0237165ce6f4120
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5831923
Auto-Submit: Koji Ishii <kojii@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1349937}