package/gcc: remove gcc 4.9

In order to reduce the number of choice in gcc selection, remove the
gcc 4.9 version.

This version was kept due to libstdc++ ABI-incompatible changes and
other build issues with kernel and bootloader as reported by Arnout
[1].

Since then, gcc 4.9 is not supported any more since glibc 2.29 [2]
and recent kernel and bootloaders has been fixed to use more recent
compiler version.

[1] http://lists.busybox.net/pipermail/buildroot/2017-June/194374.html
[2] https://www.sourceware.org/ml/libc-alpha/2019-01/msg00723.html



Signed-off-by: Romain Naour <romain.naour@smile.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>