lint: make check_precedence_confusion simpler

In C, only binary operators have possibly confusing precedence.  All
binary operators have lower precedence than an explicit cast.  When an
expression is parsed, the parentheses are associated with the innermost
possible node.  This means that as soon as a cast operator is
parenthesized, its contained expression can no longer have confusing
precedence.

This allows the code to be written more succinct since the local
variables are no longer necessary.

(rillig)