Remove '.dotnet' from SDK paths in global.json (#12880)

### Context
with this the core bootstrap does not work as intended and it confuses
copilot agents
<img width="920" height="111" alt="image"
src="https://github.com/user-attachments/assets/82792aa9-4137-4e54-a60b-85378ebf16cc"
/>


when you run the bootstrap in main with
`artifacts/bin/bootstrap/core/dotnet build` it takes Microsoft.Build.dll
from the sdk folder instead of the bootstrap folder and we're solving
that by locally deleting this line. So let's delete in the repo

### Testing
local validation it fixes the loading issue

### Notes

Author: JanProvaznik

global.json

@@ -2,7 +2,6 @@
   "sdk": {
     "allowPrerelease": true,
     "paths": [
-      ".dotnet",
       "$host$"
     ],
     "errorMessage": "The .NET SDK could not be found, please run a command-line build with ./build.cmd."