arm1176: Remove unused MMU setup from start.S

Following the removal of the smdk6400 board, the MMU setup code in
arm1176/start.S becomes unused, so remove it. It will still be possible to
restore it later from the Git history if necessary, in which case it should be
moved out of the relocate_code() function.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>