Remove dead ConcurrentDictionaryExtensions; rename memory-barrier feature flag

Delete ConcurrentDictionaryExtensions (TryRemove(KeyValuePair)) - it was added
during early MMap PR feedback for a ConcurrentDictionary-based mapping registry
that was replaced by SharedMapping/DrainReclaimer, and now has zero usages.
Also drop its companion gate FEATURE_CONCURRENTDICTIONARY_TRYREMOVE_KEYVALUEPAIR
(new in this PR, used nowhere else).

Rename FEATURE_MEMORYBARRIER_PROCESSWIDE ->
FEATURE_INTERLOCKED_MEMORYBARRIERPROCESSWIDE to match the FEATURE_<TYPE>_<MEMBER>
convention (the API is Interlocked.MemoryBarrierProcessWide), like
FEATURE_CONDITIONALWEAKTABLE_ENUMERATOR / FEATURE_STREAM_READEXACTLY.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: paulirwin

Directory.Build.targets

@@ -75,7 +75,6 @@
   <!-- Features in .NET 5.x+ only -->
   <PropertyGroup Condition=" $(TargetFramework.StartsWith('net5.')) Or $(TargetFramework.StartsWith('net6.')) Or $(TargetFramework.StartsWith('net7.')) Or $(TargetFramework.StartsWith('net8.')) Or $(TargetFramework.StartsWith('net9.')) Or $(TargetFramework.StartsWith('net10.')) ">
 
-    <DefineConstants>$(DefineConstants);FEATURE_CONCURRENTDICTIONARY_TRYREMOVE_KEYVALUEPAIR</DefineConstants>
     <DefineConstants>$(DefineConstants);FEATURE_MEMORYMARSHAL_GETARRAYDATAREFERENCE</DefineConstants>
     <DefineConstants>$(DefineConstants);FEATURE_READONLYSET</DefineConstants>
     <!-- CompareInfo.GetSortKey(ReadOnlySpan<char>, Span<byte>, CompareOptions) and GetSortKeyLength(ReadOnlySpan<char>, CompareOptions) -->
@@ -108,7 +107,7 @@
     <DefineConstants>$(DefineConstants);FEATURE_CONDITIONALWEAKTABLE_ADDORUPDATE</DefineConstants>
     <DefineConstants>$(DefineConstants);FEATURE_CONDITIONALWEAKTABLE_ENUMERATOR</DefineConstants>
     <DefineConstants>$(DefineConstants);FEATURE_ENCODING_GETSTRING_READONLYSPAN</DefineConstants>
-    <DefineConstants>$(DefineConstants);FEATURE_MEMORYBARRIER_PROCESSWIDE</DefineConstants>
+    <DefineConstants>$(DefineConstants);FEATURE_INTERLOCKED_MEMORYBARRIERPROCESSWIDE</DefineConstants>
     <DefineConstants>$(DefineConstants);FEATURE_MEMORYMARSHAL_CREATEREADONLYSPAN</DefineConstants>
     <DefineConstants>$(DefineConstants);FEATURE_NUMBER_PARSE_READONLYSPAN</DefineConstants>
     <DefineConstants>$(DefineConstants);FEATURE_QUEUE_TRYDEQUEUE_TRYPEEK</DefineConstants>

src/Lucene.Net/Support/ConcurrentDictionaryExtensions.cs

@@ -105,7 +105,7 @@ public ReadScope Enter()
             [MethodImpl(MethodImplOptions.AggressiveInlining)]
             public void EnterCore()
             {
-#if FEATURE_MEMORYBARRIER_PROCESSWIDE
+#if FEATURE_INTERLOCKED_MEMORYBARRIERPROCESSWIDE
                 // Asymmetric fencing: NO StoreLoad barrier on the hot path. Announce
                 // with a plain store (this slot is reader-private) then read _closed
                 // plain. The store and read may reorder on this core, but Close issues
@@ -225,7 +225,7 @@ public void Close(Action unmap)
             // used by hazard-pointer reclamation and the JVM's shared Arena). Without a
             // process-wide barrier the hot path fences per read instead (see Enter), so
             // a plain barrier here suffices to pair with it.
-#if FEATURE_MEMORYBARRIER_PROCESSWIDE
+#if FEATURE_INTERLOCKED_MEMORYBARRIERPROCESSWIDE
             Interlocked.MemoryBarrierProcessWide();
 #else
             Interlocked.MemoryBarrier();