Remove import using 'CoreCrossTargetingTargetsPath' property (#6668)
Remove import using 'CoreCrossTargetingTargetsPath' property (#6668)

Fixes #1062

### Context

The import based on this property was introduced as a temporary bootstrapping
mechanism before there was packaging extensibility available for multi-targeting.

The packaging mechanism (aka NuGet) now uses `buildCrossTargeting`/`buildMultiTargeting`,
similar to `build` package folder to hold and import multi-targeting logic via NuGet's Restore.

Thus, we don't need this workaround anymore.


### Changes Made

Removed the temporary import based on `CoreCrossTargetingTargetsPath`


### Testing

Since there are no tests for this property, as it is with all the workarounds… We'll see if we break anyone during the self-hosting period.


### Notes

This patch was already a part of #6161. Since, the change was approved independently, I have separated this into a new PR.
AndyGerlicher/msbuild,sean-gilliam/msbuild,AndyGerlicher/msbuild,AndyGerlicher/msbuild,rainersigwald/msbuild,sean-gilliam/msbuild,rainersigwald/msbuild,sean-gilliam/msbuild,sean-gilliam/msbuild,rainersigwald/msbuild,rainersigwald/msbuild,AndyGerlicher/msbuild