Remove unnecessary call to device_uvector::release()

[harrism]

There was an unneeded call to device_uvector::release() that did not use the return value. rapidsai/rmm#857 marks that function [[nodiscard]], so removing this allows libcudf to compile after the RMM PR is merged.

The release() is unnecessary because prev_base_offsets is passed by rvalue reference and therefore will be freed when the function exits (the uvector in the calling context it has been moved-from).

[codecov]

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@@               Coverage Diff               @@
##             branch-21.10    #9237   +/-   ##
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  Files                   ?      115           
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  Hits                    ?     2035           
  Misses                  ?    16726           
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[harrism]

@gpucibot merge