uboot-phytec

board: Remove overlooked vestiges of "dave" board.

Apparently, all "dave"-related vendor content was removed in commit
5344cc1a82fcc2817d4671696b3939b0dfa4323e; remove remaining directory
board/dave/, which consists solely of board/dave/common/flash.c.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>