Remove quotes from subshell call in tools/split.sh

Remove quotes from subshell call in tools/split.sh

Always no quotes for $() statement.

We don't need quotes to hold blanks in result:
 # i=$(echo 1 2 3)
 # echo $i
 1 2 3
 #

These quotes can make something wrong in some case:
 # i=$(echo '!')
 #
 # i="$(echo '!')"
 -bash: !: event not found
 #

No real problem for current code in split.sh, only to use a
better code style.

Change-Id: Ib86d59e10be0aca4774ad2fc656915a96b1b6c74
Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>