Remove unused import that caused the warning

[vors]

Fix this warning.

warning: use of deprecated item 'std::ascii::AsciiExt': use inherent methods instead
  --> src/parsing/syntax_set.rs:19:5
   |
19 | use std::ascii::AsciiExt;
   |     ^^^^^^^^^^^^^^^^^^^^
   |
   = note: #[warn(deprecated)] on by default

My understanding is that comment was referring to the stable toolchain. I tested it with the stable rustup toolchain and it compiles fine, so I assume it's safe to remove.

[trishume]

This'll probably force people to upgrade to the latest stable Rust, but that's fine.