Optimize, simplify FromChunkedBase64String method (#1868)

This update optimizes and simplifies the FromChunkedBase64String method,
improving both performance and memory efficiency.

### Implementation Details
Removed unnecessary manual handling of \r and \n, as
Convert.FromBase64String already handles these characters automatically.
Eliminated Where() and ToArray() operations, significantly reducing
memory allocations and improving execution efficiency.

### Benchmark Results
Performance: ~2.4x speed improvement
Memory Usage: ~70% reduction

Author: QuocDatHoang

src/DocumentFormat.OpenXml.Framework/Packaging/FlatOpcExtensions.cs

@@ -245,7 +245,6 @@ private static byte[] FromChunkedBase64String(string? chunkedBase64String)
             return Cached.Array<byte>();
         }
 
-        var base64CharArray = chunkedBase64String.Where(c => c is not '\r' and not '\n').ToArray();
-        return Convert.FromBase64CharArray(base64CharArray, 0, base64CharArray.Length);
+        return Convert.FromBase64String(chunkedBase64String);
     }
 }