Since commit 97cc29c7 (VS: Teach generators how to mark per-config
source files, 2017-04-10, v3.9.0-rc1~268^2~2) the VS generators have
known how to generate per-config sources.  We've now converted most
other code paths to support per-config sources, so drop the check
that disallows it.

This leaves only per-config support for precompiled headers and unity
build transformations, but those are optional features that can be
addressed later.

Fixes: #18233
Issue: #19789