Remove function names from some elog() calls in heapam.c.
Remove function names from some elog() calls in heapam.c.

At least one of the names was, due to a function renaming late in the
development of ON CONFLICT, wrong. Since including function names in
error messages is against the message style guide anyway, remove them
from the messages.

Discussion: 06cd426e29c1accb82885132723b8f56749ac0f4@mail.gmail.com
Backpatch: 9.5, where ON CONFLICT was introduced

zeroae/postgres-xl,zeroae/postgres-xl,zeroae/postgres-xl,zeroae/postgres-xl,zeroae/postgres-xl