Remove oudated comment in seqs.rs

[ahuoguo]

From #1317, seq! with more than one element now doesn't encode to .push, which makes the comments here false.

Sidenote: the last test in seqs.rs:

            // Test for successful broadcast of push_distributes_over_add
            assert((s2 + s4).push(120) == s2 + s4.push(120));

doesn't seem to be a useful test sine this assertion still verifies if push_distributes_over_add is removed from group_seq_lib_default. It verifies with the following axiom-usage-info:

note: checking this function used these broadcasted lemmas and broadcast groups:
        - (group) vstd::seq::group_seq_axioms,
        - (group) vstd::seq_lib::group_seq_lib_default,
        - (group) vstd::group_vstd_default,
        - vstd::seq::axiom_seq_push_len,
        - vstd::seq::axiom_seq_push_index_same,
        - vstd::seq::axiom_seq_push_index_different,
        - vstd::seq::axiom_seq_ext_equal,
        - vstd::seq::axiom_seq_add_len,
        - vstd::seq::axiom_seq_add_index1,
        - vstd::seq::axiom_seq_add_index2,
        - vstd::seq_lib::impl&%0::filter_lemma,
        - vstd::seq_lib::impl&%0::filter_distributes_over_add

By submitting this pull request, I confirm that my contribution is made under the terms of the MIT license.

[utaal]

Is the side-note relevant to this PR? Perhaps add a TODO in the PR with the comment, so we can keep track of it, if it's not something we want to handle in this PR.