fs_mgr_fstab: removing fs_mgr_get_entry_for_mount_point_after()

No fs_mgr client is using fs_mgr_get_entry_for_mount_point_after(), which
supports matching a mount point iteratively by specifying a starting entry
in the fstab. So removes it.

Also switch the public API from C/C++ to C++-only:
    fs_mgr_get_entry_for_mount_point(struct fstab* fstab, const char* path);
==> fs_mgr_get_entry_for_mount_point(struct fstab* fstab, const std::string& path);

Bug: 67024518
Test: boot sailfish, manually test fs_mgr_get_entry_for_mount_point().

Change-Id: I38e18fa6bd9485b6cd31b55474406a511df3f506