s390/startup: purge obsolete .gitignore patterns

sizes.h and vmlinux.scr.lds are not generated since commit 369f91c3
("s390/decompressor: rework uncompressed image info collection").

vmlinux.bin.full is not generated since commit 183ab05f

 ("s390: get
rid of the first mb of uncompressed image").

Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>