Retire a few backwards compatibility macros.

Retire a few backwards compatibility macros.

As of commits dd04e958c8 and 1833f1a1c3, tuplestore_donestoring(),
SPI_push(), SPI_pop(), SPI_push_conditional(),
SPI_pop_conditional(), and SPI_restore_connection() are no-op
macros provided for backwards compatibility.  This commit removes
these macros, so any uses in third-party code will need to be
removed, too.  Since these macros have been no-ops for a while,
such adjustments won't produce any behavior changes for all
currently-supported versions of PostgreSQL.

Author: Bharath Rupireddy
Discussion: https://postgr.es/m/CALj2ACVeO58JM5tK2Qa8QC-%3DkC8sdkJOTd4BFU%3DK8zs4gGYpjQ%40mail.gmail.com