remove auto-preview (#7344)

Author: KevinRansom

src/fsharp/FSharp.Build/Microsoft.FSharp.Targets

@@ -272,13 +272,6 @@ this file.
             <FscOtherFlags Condition="'$(SimpleResolution)' != 'true'">$(OtherFlags)</FscOtherFlags>
         </PropertyGroup>
 
-        <PropertyGroup>
-            <!-- default to preview when building netcoreap3.0 or netstandard2.1 -->
-            <LangVersion Condition="'$(LangVersion)' == '' AND
-                (('$(TargetFrameworkIdentifier)' == '.NETCoreApp' AND '$(TargetFrameworkVersion)' == 'v3.0') OR
-                ('$(TargetFrameworkIdentifier)' == '.NETStandard' AND '$(TargetFrameworkVersion)' == 'v2.1'))">preview</LangVersion>
-        </PropertyGroup>
-
         <!-- Condition is to filter out the _CoreCompileResourceInputs so that it doesn't pass in culture resources to the compiler -->
         <!-- NOTE: ManifestResourceWithNoCulture and ManifestNonResxWithNoCultureOnDisk are generated by Mono targets files -->
         <Fsc  Condition="'%(_CoreCompileResourceInputs.WithCulture)' != 'true'"