kernel.bbclass: remove empty module directories to prevent QA issues

Currently, allyesconfig test runs for x86_64 fail with:
ERROR: linux-yocto-5.19.17+gitAUTOINC+0cba9aa404_aaf4490d18-r0 do_package:
QA Issue: linux-yocto: Files/directories were installed but not shipped in any
package:
  /lib/modules/5.19.17/kernel/drivers/nvdimm

With CONFIG_NVDIMM_TEST_BUILD=m, an empty nvdimm directory is created during
modules_install, which triggers the QA issue.

Extend kernel_do_install() to also remove inner empty directories that might
get created by modules_install.

Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>