sql-server and t-sql (short for Transact-SQL) both represent practically the same thing, which is Microsoft's proprietary RDBMS.

From Wikipedia:

Transact-SQL (T-SQL) is Microsoft's and Sybase's proprietary extension to SQL. [...]

Transact-SQL is central to using Microsoft SQL Server. All applications that communicate with an instance of SQL Server do so by sending Transact-SQL statements to the server, regardless of the user interface of the application.

As you can see, there is a lot of overlap...

If we were to keep one or the other, I think t-sql would be better to keep, as it is the language, rather than the technology/DB engine that is sql-server.