kernel-headers: remove deprecated versions 3.1, 3.3, 3.5

The Linux kernel headers versions 3.1, 3.3, and 3.5 have been deprecated
since 2013.02 and thus can be removed in 2014.02.
For legacy handling, we automatically select versions 3.2, 3.4, and 3.6,
respectively.

Additionally, this patch removes the now unused symbol
BR2_DEPRECATED_SINCE_2013_02.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>