binutils: remove 2.21.1

After the following commits:

26974bf6 use default binutils for sh architecture
6f1cf344 binutils: Use the default version by default on MIPS

the 2.21.1 binutils version is no longer required, and since it's an
old version, we get rid of it.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>