Bluetooth: shell: kconfig: Do not select FLASH_SHELL from BT_SHELL

Fixes some selects with unsatisfied dependencies. FLASH_SHELL depends on
FLASH_PAGE_LAYOUT, but only FLASH_SHELL is selected. The select was
introduced in commit 3aa2a1c6 ("flash: make flash shell generic").

Spoke to Johan Hedberg. The BT_SHELL code does not depend on
FLASH_SHELL. He didn't think people would assume that BT_SHELL by itself
enables the FLASH_SHELL commands either.

Just remove the select.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>