[dev.link] cmd/link/internal/loader: get rid of the AttrContainer loader method

Remove the loader's AttrContainer method, since it is no longer
needed. All of the code in the linker that used s.Attr.Container() is
now upstream of loadlibfull(), and the code in question now uses local
bitmaps to keep track of container text symbols as opposed to loader
methods.

Change-Id: Iae956d24bef2776e181c3b8208476dcb0b9a2916
Reviewed-on: https://go-review.googlesource.com/c/go/+/227959


Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>