cmake --build: Use both inter- and intra-project parallelism with msbuild

Since commit 1ab3881ec9 (cmake: Add options for parallel builds to
--build mode, 2018-04-14, v3.12.0-rc1~42^2), `cmake --build --parallel`
has added two options to the msbuild command line:

* `/m` enables inter-project parallelism in msbuild.
* `/p:CL_MPCount=1` suppresses intra-project parallelism in cl.

The latter was used to avoid `O(N^2)` compilation threads on `N`
processors.  In practice however, projects often have many source files
in a few targets, so `--parallel` actually reduces parallelism.

Drop the latter option to restore intra-project parallelism.
Users can always add `-- /p:CL_MPCount=1` themselves.

Fixes: #20564