remove dead and/or broken TT_SEQUENCE case from parse_pagenode

For context, this part of the code is concerned with parsing a content stream
as specified by the /Contents entry of a page node. The section being removed
by this commit is trying to deal with the case that /Contents is an array
(represented as a sequence token by the parser). It was originally introduced
in commit 285b6f89 as the else branch of an 'if' that checked for token type
TT_Objstm. The sequence case was apparently found to be buggy, so the next
commit, ddf109c3, introduced a different case for TT_SEQUENCE that does
nothing, leaving this as the new else.

So this code is certainly broken (it tries to access a sequence if and only if
that token is not a sequence) and unreachable (no token types appear beside
TT_Objstm and TT_SEQUENCE) if we are lucky. As noted correctly by the preceding
comment introduced with ddf109c3, this bit of code is at best an incomplete
version of a later piece that attempts to accomplish the same thing. We will
come back to that one.