Remove dead BUILDING_WITH_LKG define

[bbatsov]

PR #19235 removed all the #if BUILDING_WITH_LKG conditionals from source code, but left the constant itself defined in Directory.Build.props and FSharp.Profiles.props. It's dead code now — no #if directive references it. This removes both definitions.

Fixes #18061

[github-actions]

✅ No release notes required

[T-Gro]

I think we shall keep it - to make life of future contributors, human or agentic, easier.

The constant is by nature ephemeral.
We only ever need it when we create a new language feature, want to dogfood such feature, and we know LKG (Last Known Good, an acronym to indicate a production shipped version of the compiler) does not support it.
As of now, no such feature exists, but I think it is a matter of time ( #18927 comes to mind :))) ).

While the feature is developed, we will then again need to temporarily have #if BUILDING_WITH_LKG in the code, block without the feature for LKG, and then a "modern" code block that uses the bootstrapped compiler.
And after the feature is shipped to a stable LKG, we can again remove the #if.

One can wonder - why not just wait until the feature gets shipped?
It has proven value during development to apply the feature immediately, because it is a good source of dogfooding - not only for bugs, but also for "testing" ergonomics and.

[bbatsov]

Up to you - I was just going over issues with "good first issue" and this one seemed simple enough.

[T-Gro]

The issue about nullness related ifdefs is completed now, thanks for bringing it to my attention.
Lets keep the BUILDING_WITH_LKG in 👍 .