remove setting of the seekable field

remove setting of the seekable field, since it causes access violation errors, and unnecessary.

Check the comment from june 6, 2023: unosquare#642

Or:
Ruslan-B/FFmpeg.AutoGen#255

Currently I'm uusing my own package, but I'd liek to swicth your original version, but this issue block me.

Reproduce the issue: Just open any local video file with the following url:
customfile://C:\myfolder\myfile.mkv
It will use the FileInputStream which sets the seekable field. => access violation when trying to open the media.

Unosquare.FFME/Container/MediaContainer.cs

@@ -682,9 +682,6 @@ private void StreamInitialize()
                     CustomInputStreamContext = ffmpeg.avio_alloc_context(
                         inputBuffer, CustomInputStream.ReadBufferLength, 0, null, CustomInputStreamRead, null, CustomInputStreamSeek);
 
-                    // Set the seekable flag based on the custom input stream implementation
-                    CustomInputStreamContext->seekable = CustomInputStream.CanSeek ? ffmpeg.AVIO_SEEKABLE_NORMAL : 0;
-
                     // Assign the AVIOContext to the input context
                     inputContextPtr->pb = CustomInputStreamContext;
                 }