src/meta-raspberrypi:linux: drop unused 5.15 version

* 6.1 version was used by default since mickledore:
  agherzan/meta-raspberrypi@12a1187
  and kirkstone for rpi5 MACHINEs since:
  agherzan/meta-raspberrypi@9dc6673

* scarthgap uses 6.6 by default since:
  agherzan/meta-raspberrypi@1cf3dd5

* nobody is probably using 5.15 with newer release and there are issues with "iw reg" since
  wireless-regdb upgrade in oe-core as well as shown in:
  agherzan/meta-raspberrypi#1317
  so better to drop it from master/scarthgap together with
  rpi.scc used only for 5.15 and 0001-gcc-plugins-Reorganize-gimple-includes-for-GCC-13.patch
  used only by rpi.scc

Signed-off-by: Martin Jansa <martin.jansagmail.com>