Revert "wic: set bootimg_dir when using image-name artifacts"

Revert "wic: set bootimg_dir when using image-name artifacts"

This reverts commit 7ce1dc13f91df70e8a2f420e7c3eba51cbc4bd48.

This patch broke the assumption that a non-null boot_dir means a
user-assigned (-b command-line param) value.

Reverting doesn't break anything, since the case it was added for
doesn't use the boot_dir for anything except debugging anyhow.

Fixes [YOCTO #6290]

(From OE-Core rev: db90f10bf31dec8d7d7bb2d3680d50e133662850)

(From OE-Core rev: 36c93423ee272c4d4aafeb50f83734fd4bb3bb29)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Armin Kuster <akuster808@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Patrick Ohly <patrick.ohly@intel.com>