Very good correct Lowell!
Here's one that is slicker:
EXEC LinkedServer.yourdb.sys.sp_executesql N'DROP USER myuser'
Not the least is this one slick, if any of the items are variable:
DECLARE @sp_executesql nvarchar(1024) =
quotename(@server) + '.' + quotename(@db) + '.sys.sp_executesql'
EXEC @sp_executesql N'DROP USER myuser'
Key here is that EXEC accepts a variable for the procedure name, and a system procedure always executes in the context of the database from which it is invoked.