ARC: configs: Remove CONFIG_INITRAMFS_SOURCE from defconfigs

ARC: configs: Remove CONFIG_INITRAMFS_SOURCE from defconfigs

We used to have pre-set CONFIG_INITRAMFS_SOURCE with local path
to intramfs in ARC defconfigs. This was quite convenient for
in-house development but not that convenient for newcomers
who obviusly don't have folders like "arc_initramfs" next to
the Linux source tree. Which leads to quite surprising failure
of defconfig building:
------------------------------->8-----------------------------
  ../scripts/gen_initramfs_list.sh: Cannot open '../../arc_initramfs_hs/'
../usr/Makefile:57: recipe for target 'usr/initramfs_data.cpio.gz' failed
make[2]: *** [usr/initramfs_data.cpio.gz] Error 1
------------------------------->8-----------------------------

So now when more and more people start to deal with our defconfigs
let's make their life easier with removal of CONFIG_INITRAMFS_SOURCE.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Kevin Hilman <khilman@baylibre.com>
Cc: stable@vger.kernel.org
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>