gcc: remove versions 4.3.x and 4.6.x

Those gcc series are old and are not used as the default versions for
any of the architectures we support, so this commit gets rid of them.

The gcc 4.3.x series technically remains used by the LPC32xx
defconfigs we have:

configs/ea3250_defconfig:BR2_GCC_VERSION_4_3_X=y
configs/fdi3250_defconfig:BR2_GCC_VERSION_4_3_X=y
configs/phy3250_defconfig:BR2_GCC_VERSION_4_3_X=y

Back when those defconfigs were introduced, gcc 4.3 was chosen because
it was the only one capable of building a fully working kernel for
those ARM-based platforms. However, the original submitter, Alexandre
Belloni, no longer has access to the hardware platforms, so he is
unable to test if newer gcc versions have fixed the problem. It
certainly doesn't make sense to keep gcc 4.3.x just for those three
boards, so we'll wait for someone actually using those defconfigs to
complain.

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