VS: Revert "Build custom commands concurrently when possible"

Since commit 33c15ae2b9 (VS: Build custom commands concurrently when
possible, 2023-01-19, v3.26.0-rc1~56^2) we add `BuildInParallel` to
custom commands in `.vcxproj` files.  However, this can break existing
projects that implicitly rely on serial execution of custom commands.
For example, custom commands in our FindCUDA module run MSVC (via nvcc)
with a common `vc*.pdb` file, and therefore cannot run in parallel.

Revert use of `BuildInParallel` while leaving most of the infrastructure
for it in place.  It can be restored later with an option or policy.

Fixes: #24576
Issue: #18405