Remove unused and broken mtd_write_img function from libmtd

Remove unused and broken mtd_write_img function from libmtd

The function _tries_ to support short reads but doesn't adjust the
pointer into the buffer. If a short read happens, we scrambles the
flash contents. Interrupted reads aren't handled. Short or
interrupted writes aren't handled at all. Either a write succeeds
writing the entire buffer or the function gives up.

During an attempt at fixing it, it was discovered, that no mtd-utils
program uses this function. Furthermore, its highly specific nature
makes it more of a "feature looking for use case".

Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>
Signed-off-by: Richard Weinberger <richard@nod.at>