Task: Remove .NET Standard 1.x/.NET Core 1.x build features

[NightOwl888]

There are several features that are still hanging around even though we dropped support for .NET Core 1.x and .NET Standard 1.x years ago. These can be removed, which will help us simplify maintenance.

ICU4N/Directory.Build.targets

Lines 133 to 149 in 88a0288

	<!-- Features in .NET Standard 1.x and .NET Core 1.x -->
	<PropertyGroup Condition=" $(TargetFramework.StartsWith('netstandard1.')) Or $(TargetFramework.StartsWith('netcoreapp1.')) ">
	<DefineConstants>$(DefineConstants);NETSTANDARD1_X</DefineConstants>
	<DefineConstants>$(DefineConstants);NETCOREAPP1_X</DefineConstants>
	<DefineConstants>$(DefineConstants);FEATURE_CULTUREINFO_UNKNOWNLANGUAGE</DefineConstants>
	<DefineConstants>$(DefineConstants);FEATURE_TYPEEXTENSIONS_GETTYPEINFO</DefineConstants>
	<DefineConstants>$(DefineConstants);FEATURE_CULTUREINFO_DEFAULTTHREADCURRENTCULTURE</DefineConstants>
	<DefineConstants>$(DefineConstants);FEATURE_EXCEPTION_HRESULT</DefineConstants>
	<DefineConstants>$(DefineConstants);FEATURE_MICROSOFT_EXTENSIONS_CACHING</DefineConstants>
	<DefineConstants>$(DefineConstants);FEATURE_IREADONLYCOLLECTIONS</DefineConstants>
	<DefineConstants>$(DefineConstants);FEATURE_REGEX_MATCHTIMEOUT</DefineConstants>
	<DefineConstants>$(DefineConstants);FEATURE_TASK_ASYNC_AWAIT</DefineConstants>
	<DefineConstants>$(DefineConstants);FEATURE_TASK_RUN</DefineConstants>
	<DefineConstants>$(DefineConstants);FEATURE_TYPEDWEAKREFERENCE</DefineConstants>
	</PropertyGroup>