Remove unused locals in System.Data.OleDb (dotnet/corefx#42485)

Remove unused locals in System.Data.OleDb (dotnet/corefx#42485)

* emove unused locals in System.Data.OleDband keep some logic using discard. Partially fix issue dotnet/corefx#39962

* Update src/System.Data.OleDb/src/OleDbDataReader.cs

update as suggested (OleDbDataReader)

Co-Authored-By: Stephen Toub <stoub@microsoft.com>
* Update src/System.Data.OleDb/src/OleDbTransaction.cs

Done update as suggested

Co-Authored-By: Stephen Toub <stoub@microsoft.com>
* Update src/System.Data.OleDb/src/RowBinding.cs

Done update as suggested (RowDataBinding)

Co-Authored-By: Stephen Toub <stoub@microsoft.com>
* Update src/System.Data.OleDb/src/RowBinding.cs

Done as suggested

Co-Authored-By: Stephen Toub <stoub@microsoft.com>
* Update src/System.Data.OleDb/src/SafeHandles.cs

Done updating as suggested (SafeHandles)

Co-Authored-By: Stephen Toub <stoub@microsoft.com>
Commit migrated from https://github.com/dotnet/corefx/commit/eceea8b4284ca42d8bd0134414b43c835bd80e4c