riscv: Delete CONFIG_SYSFS_SYSCALL from defconfigs

According to init/Kconfig:
"sys_sysfs is an obsolete system call no longer supported in libc.
Note that disabling this option is more secure but might break
compatibility with some systems."

This syscall is not required for new architectures. Since the config
defaults to 'y'. Set this to 'n' exlicitly.

Signed-off-by: Deepa Dinamani <deepa.kernel@gmail.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>