remove redundant call to index_truncate in Container_fd::trunc.
this calls containerfs_zero_helper() that does the trucate, so it
doesn't need to do the call.

the containerfs_zero_helper() handles three cases (file open --
Container_fd::trunc, file closed -- ContainerFileSystem::trunc,
and file being opened with O_TRUNC).  it makes sense to have the
helper always handle the truncate for all cases so at at high
level it does the same thing (i.e. handle the index).