Also various related cleanup in ParseVariableDeclarations(). The only
changes in logic are explained below:

  - We were redundantly checking for parenthesized binding patterns;
    these are already ruled out by BindingPatternUnexpectedToken()
    calls in the places where we hit an LPAREN.
  - There's no need to default-initialize a LET-mode variable in a
    for-each loop, just as there isn't for CONST or CONST_LEGACY
    (ParseForStatement will take care of properly initializing all
    of the above).

Review URL: https://codereview.chromium.org/1661193002

Cr-Commit-Position: refs/heads/master@{#33749}