Parser: remove the use_lhs parser flag

Parser: remove the use_lhs parser flag

Previously, use_lhs told njs_parser_exponentiation_expression() to skip the
unary part and njs_parser_assignment_expression() to skip the arrow check, so
that an expression could resume from an already parsed left-hand side.

The fix is to remove it: its only writer went with the speculative "for"
initializer parse.