populate_sdk: don't force target debug packages into SDK
TOOLCHAIN_TARGET_TASK doesn't need to explicitly list sdk-target-dbg because if
SDKIMAGE_FEATURES contains dbg-pkgs (as it does by default) then they'll all be
installed anyway. This means that if the user removes dbg-pkgs from
SDKIMAGE_FEATURES then the SDK correctly doesn't have debug packages in.
[ YOCTO #9078 ]
Signed-off-by: Ross Burton <ross.burton@intel.com>