docs/manual: update external toolchain advantages list

Buildroot can build non-uClibc toolchains internally for quite some time now.
Update the manual text.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

docs/manual/configure.txt

@@ -221,8 +221,6 @@ Advantages of this backend:
   often very significant in the overall build time of an embedded
   Linux system.
 
-* Not limited to uClibc: glibc and eglibc toolchains are supported.
-
 Drawbacks of this backend:
 
 * If your pre-built external toolchain has a bug, may be hard to get a