It is not necessary to link 'app' with ELMFAT or NFFS because their
usage requirements are covered by the 'APP_LINK_WITH_MBEDTLS'
mechanism that automatically links 'app' with the interface library
FS.
This patch removes the redundant target_link_libraries invocations.
Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>