Remove the prohibition on executing cursor commands through SPI_execute.

Remove the prohibition on executing cursor commands through SPI_execute.
Vadim had included this restriction in the original design of the SPI code,
but I'm darned if I can see a reason for it.

I left the macro definition of SPI_ERROR_CURSOR in place, so as not to
needlessly break any SPI callers that are checking for it, but that code
will never actually be returned anymore.