exit_group.2: Remove a confusing reference to _exit(2) in DESCRIPTION

As noted by Jakub:

    BTW, the exit_group.2 man page could use an update (possibly
    by merging it into exit.2): it says that the "system
    call is is equivalent to _exit(2) except that it terminates
    not only the calling thread, but all threads in the calling
    process's thread group", which isn't helpful these days.

Reported-by: Jakub Wilk <jwilk@jwilk.net>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/exit_group.2

@@ -39,9 +39,7 @@ glibc provides no wrapper for
 necessitating the use of
 .BR syscall (2).
 .SH DESCRIPTION
-This system call is equivalent to
-.BR _exit (2)
-except that it terminates not only the calling thread, but all threads
+This system call terminates all threads
 in the calling process's thread group.
 .SH RETURN VALUE
 This system call does not return.