fs_mgr: Remove deprecated include path system/core/fs_mgr/include_fstab

These headers are exported by libfs_mgr and libfstab. Users should use
those libraries instead of directly referencing the include dir with
LOCAL_C_INCLUDES.

Bug: 293695109
Test: presubmit build pass
Change-Id: I888f59879f31fb75ddd5a0d243d4796415b57c7d