Fix errors with 2+ GPUs and simplify total speed calculation

[zZoMROT]

Fixed errors occurring when using 2 or more GPUs:

• Segmentation fault
• free(): invalid pointer
• double free or corruption (fasttop)
• Aborted (core dumped)

Removed unnecessary handling of samples for total speed. Now, the total speed is calculated as the sum of the speeds of individual GPUs, simplifying the code and improving stability.

Related to https://github.com/johguse/ERADICATE2/issues/6