confis/qemu_mips64r6*_malta: use the default compiler version (gcc 8.x)

Theses defconfig was added at the when gcc 6 was not the default version
used by Buildroot, so the gcc version was explicitely set.

Since then, gcc 8.x is used as the default. So drop BR2_GCC_VERSION_6_X.

Tested with Qemu 2.8.1 (Debian 1:2.8+dfsg-6+deb9u7)
qemu_mips32r6_malta_defconfig
qemu_mips32r6el_malta_defconfig
qemu_mips64r6_malta_defconfig
qemu_mips64r6el_malta_defconfig

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