Revert WAL posix_fallocate() patches.

This reverts commit 269e780822abb2e44189afaccd6b0ee7aefa7ddd
and commit 5b571bb8c8d2bea610e01ae1ee7bc05adcfff528.

Unfortunately, the initial patch had insufficient performance testing,
and resulted in a regression.

Per report by Thom Brown.