hush: Remove default CONFIG_SYS_PROMPT_HUSH_PS2 setting from board files

There is no reason to define default option for this macro which is
already done in common/cli_hush.c.

  87 #ifndef CONFIG_SYS_PROMPT_HUSH_PS2
  88 #define CONFIG_SYS_PROMPT_HUSH_PS2      "> "
  89 #endif

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Acked-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>