Remove last traces of heap_open/close in the tree

Remove last traces of heap_open/close in the tree

Since pluggable storage has been introduced, those two routines have
been replaced by table_open/close, with some compatibility macros still
present to allow extensions to compile correctly with v12.

Some code paths using the old routines still remained, so replace them.
Based on the discussion done, the consensus reached is that it is better
to remove those compatibility macros so as nothing new uses the old
routines, so remove also the compatibility macros.

Discussion: https://postgr.es/m/20191017014706.GF5605@paquier.xyz