Remove duplicate InitPostmasterChild() call while starting a bgworker.

This is apparently harmless on Windows, but on Unix it results in an
assertion failure.  We'd not noticed because this code doesn't get
used on Unix unless you build with -DEXEC_BACKEND.  Bug was evidently
introduced by sloppy refactoring in commit 31c45316.

Thomas Munro

Discussion: <CAEepm=1VOnbVx4wsgQFvj94hu9jVt2nVabCr7QiooUSvPJXkgQ@mail.gmail.com>