Remove tcb_size and flag argument for _rtld_allocate_tls,

Remove tcb_size and flag argument for _rtld_allocate_tls,
remove tcb_size argument for _rtld_free_tls. The size is defined
by the architecture's struct tls_tcb layout and the behaviour
of RTLD_ALLOC_TLS_FREE_OLD is the only possible reaction for
old_tcb != NULL which doesn't leak memory.