env: ENV_IS_IN_FAT improvements

Make it select FS_FAT as well, because if it's not selected, enabling
ENV_IS_IN_FAT causes a Kconfig warning:

warning: (ENV_IS_IN_FAT) selects FAT_WRITE which has unmet direct dependencies (FS_FAT)

This also allows dropping some code from config_fallbacks.

Also drop the unnecessary help text about having to enable
CONFIG_FAT_WRITE - Kconfig automatically handles that.

Signed-off-by: Tuomas Tynkkynen <tuomas@tuxera.com>