kconfig: remove redundant "string" type in arch and board Kconfigs

Now the types of CONFIG_SYS_{ARCH, CPU, SOC, VENDOR, BOARD, CONFIG_NAME}
are specified in arch/Kconfig.

We can delete the ones in arch and board Kconfig files.

This commit can be easily reproduced by the following command:

find . -name Kconfig -a ! -path ./arch/Kconfig | xargs sed -i -e '
/config[[:space:]]SYS_\(ARCH\|CPU\|SOC\|\VENDOR\|BOARD\|CONFIG_NAME\)/ {
    N
    s/\n[[:space:]]*string//
}
'

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>