sh: Cleanup about SPARSE_IRQ

After commit 37744fee ("sh: remove sh5 support"), sh always
enable SPARSE_IRQ, kill unused MAY_HAVE_SPARSE_IRQ and NR_IRQS
define.

Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Rich Felker <dalias@libc.org>