double call of flash_page_erase for copy_page use in file system code

[kevinjwalters]

In microbitfs.c there's a single call to copy_page() with a preceeding called to flash_page_erase().

micropython-microbit-v2/src/codal_port/microbitfs.c

Lines 235 to 240 in 7c304ec

	while (page != end_page) {
	uint8_t *next_page = page+step;
	flash_page_erase((uint32_t)page);
	copy_page(page, next_page);
	page = next_page;
	}

Inside copy_page() it also calls flash_page_erase() on first arg.

micropython-microbit-v2/src/codal_port/microbitfs.c

Lines 194 to 196 in 7c304ec

	static void copy_page(void *dest, void *src) {
	DEBUG(("FILE DEBUG: Copying page from %lx to %lx.\r\n", (uint32_t)src, (uint32_t)dest));
	flash_page_erase((uint32_t)dest);

Unless there's some magic under the covers I think this will be causing unnecessary wear on the flash? And maybe slowing things down a tiny bit?