drivers: flash: stm32: don't flush i-cache after an erase

In general synchronisation between memory and the instruction cache
should be done explicitly, for example this is how it is done for RAM.
It is also done that way for flash writes in the current STM32 flash
driver.

However in case of flash erases, the current STM32 flash driver flushes
the i-cache. It probably doesn't make a big difference for the
Cortex-M3/M4 families as the cache is very small. That said it might
have bigger impact on families based on Cortex-M7 as it has between 4KB
and 16KB of i-cache, that will be added latter in this patch serie.

This patch therefore remove the i-cache flush in
flash_stm32_flush_caches.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>