Lexer: delete dead code for binary character literals

The Haskell 2010 report chapter 2.6 (Characters and String Literals)
says:

  "Numeric escapes such as \137 are used to designate the character with
  decimal representation 137; octal (e.g. \o137) and hexadecimal (e.g.
  \x37) representations are also allowed."

Commit 1c0b5fdc9f2b6ea8166cc565383d4cd20432343c added syntax for writing
character literals using binary notation (e.g. '\b100100'). But this
code can never be reached, because '\b' already represents "backspace".

Turn on -fwarn-overlapping-patterns to catch such bugs in the future.

Reviewed by: hvr

Differential Revision: https://phabricator.haskell.org/D1291

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