doc: remove "known bug" about global variables in debug_info.

This hasn't been true at least since 1.4. Until golang.org/cl/137235
they were lumped together into a random compile unit, now they are
assigned to the correct one.

Change-Id: Ib66539bd67af3e9daeecac8bf5f32c10e62e11b1
Reviewed-on: https://go-review.googlesource.com/138415Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: David Chase <drchase@google.com>

doc/debugging_with_gdb.html

@@ -179,7 +179,6 @@ from it.</li>
 <code>"fmt.Print"</code> as an unstructured literal with a <code>"."</code>
 that needs to be quoted.  It objects even more strongly to method names of
 the form <code>pkg.(*MyType).Meth</code>.
-<li>All global variables are lumped into package <code>"main"</code>.</li>
 <li>As of Go 1.11, debug information is compressed by default.
 Older versions of gdb, such as the one available by default on MacOS,
 do not understand the compression.