unused variable ShmemBootstrap

Patchers,

src/backend/storage/shmem.c contains an unused variable; it's only
assigned to, and used in a couple Assert(). It's been this way since
shmem.c 1.53, which was current on july 2000.

This patch removes it, along with an obsolete comment (which refers to a
file and function that hasn't existed at all in the whole CVS history.)

--
Alvaro Herrera (<alvherre[(at)]dcc(dot)uchile(dot)cl>)
"There is evil in the world. There are dark, awful things. Occasionally, we get
a glimpse of them. But there are dark corners; horrors almost impossible to
imagine... even in our worst nightmares." (Van Helsing, Dracula A.D. 1972)