This code used to be necessary because of the error messages generated
by the YACC-based parser, but they're no longer relevant under the new
recursive descent parser:

  - LBRACE no longer exists, so "{ or {" can never occur.

  - The parser never generates error messages about "@" or "?" now
    (except in import sections, where they're actually legitimate).

  - The s/LLITERAL/litbuf/ substitution is handled in p.syntax_error.

Change-Id: Id39f747e4aa492c5830d14a47b161920bd4589ad
Reviewed-on: https://go-review.googlesource.com/17690Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>