Remove UnitySDK.log file

[andrewcoh]

This change removes the feature that Academy.cs creates and writes to UnitySDK.log in the directory of the build. UnitySDK.log is a bit redundant given the player log and as written it's logging of "System.DateTime.Now" was causing the following warning and exception with newer versions of Unity.

Warning
Plugins: Couldn't open libc, error: dlopen(libc, 2): image not found

Exception
ObjectDisposedException: Cannot write to a closed TextWriter.
System.IO.StreamWriter.Flush (System.Boolean flushStream, System.Boolean flushEncoder) (at <b35a51e37b9044f3a540f83fa39a96e3>:0)
System.IO.StreamWriter.Write (System.Char[] buffer, System.Int32 index, System.Int32 count) (at <b35a51e37b9044f3a540f83fa39a96e3>:0)
System.IO.TextWriter.WriteLine (System.String value) (at <b35a51e37b9044f3a540f83fa39a96e3>:0)
MLAgents.Academy.InitializeEnvironment () (at <31b28fc73f93469aa66b0663de536fb0>:0)
MLAgents.Academy.Awake () (at <31b28fc73f93469aa66b0663de536fb0>:0)

This change also modifies the UnityTimeOutException class to simply pass the exception message through to Exception (as most of the other exceptions do) and no longer read from UnitySDK.log.

[vincentpierre]

I approve : 🚢🇮🇹
Are you sure the UnitySDK.log file is not referenced anywhere in the documentation? If it is, you should change the document in this PR.