malloc_f: remove redundant defalut values of CONFIG_SYS_MALLOC_F_LEN

The default value of CONFIG_SYS_MALLOC_F_LEN is defined by ./Kconfig
as 0x400.  Each defconfig or Kconfig need not repeat the same value.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Robert Baldyga <r.baldyga@samsung.com>