remove unnecessary variables and delimiter check

[bb1yd]

Zulip discussion: https://rust-lang.zulipchat.com/#narrow/channel/182449-t-compiler.2Fhelp/topic/question.20about.20lex_token_tree_open_delim.20in.20rustc_parse/with/580552157

In lex_token_tree_open_delim, after calling self.lex_token_trees(/* is_delimited */ true):

• If the delimiters are well-balanced, the value popped from the stack will always equal to open_delim and pre_span.
• If the delimiters are not balanced, self.lex_token_trees(/* is_delimited */ true) will return an error so this branch will not be reached

Therefore open_delimiter and open_delimiter_span can be removed.

Test tests/ui/parser all passed

[rustbot]

The parser was modified, potentially altering the grammar of (stable) Rust
which would be a breaking change.

cc @fmease

[rustbot]

r? @jackh726

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