doc: remove duplicate -asmflags mention

Fixes #12053

Change-Id: Icd883b4f1ac944a8ec718c79770a8e3fc6542e3a
Reviewed-on: https://go-review.googlesource.com/13259Reviewed-by: Russ Cox <rsc@golang.org>

doc/go1.5.html

@@ -522,14 +522,6 @@ A new <code>-buildmode</code> build option
 sets the build mode, described above.
 </li>
 
-<li>
-An <code>-asmflags</code> build option has been added to provide
-flags to the assembler.
-However,
-the <code>-ccflags</code> build option has been dropped;
-it was specific to the old, now deleted C compiler .
-</li>
-
 <li>
 A new <code>-pkgdir</code> build option
 sets the location of installed package archives,