mgmt/mcumgr/lib: Merge Zephyr specific code to fs_mgmt.c

It is no longer needed to have "system specific" API for FS management.
The commit removes fs_mgmt_impl.h and moves Zephyr specific
functions that implement the fs_mgmt_impl.h declared functions
into fs_mgmt.c

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>