drivers/: Make 'fallback_{un}map' the default unless defined

Drop the explicit need to specify the default 'fallback_{un}map'
callback function pointer from the 'programmer_entry' struct.
This is a reasonable default for every other driver in the tree
with only a select few exceptions [atavia, serprog, dummyflasher
and internal].

Thus this simplifies driver development and paves way
to remove the 'programmer' global handle.

Change-Id: I5ea7bd68f7ae2cd4af9902ef07255ab6ce0bfdb3
Signed-off-by: Edward O'Callaghan <quasisec@google.com>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/67404
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Singer <felixsinger@posteo.net>
Reviewed-by: Anastasia Klimchuk <aklm@chromium.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

serprog.c

@@ -967,6 +967,5 @@ const struct programmer_entry programmer_serprog = {
 	.devs.note		= "All programmer devices speaking the serprog protocol\n",
 	.init			= serprog_init,
 	.map_flash_region	= serprog_map,
-	.unmap_flash_region	= fallback_unmap,
 	.delay			= serprog_delay,
 };