[NFC][analyzer] Cleanup dead code around NodeBuilder (#179711)

As I was trying to understand the class `NodeBuilder` and its
subclasses, I wasted a few hours on studying dead or needlessly
complicated code. I'm creating this patch to ensure that others in the
future won't need to bother with this cruft.

This commit eliminates three deficiencies:
- (Small change:) In a constructor of `StmtNodeBuilder` I switched to
using the `takeNodes()` overload which accepts an `ExplodedNodeSet`
(instead of manually iterating).
- The `Finalized` attribute of NodeBuilder was completely irrelevant (it
was always initialized to `true`).
- The "main" feature of `NodeBuilderWithSinks` was that it gathered the
generated sink nodes into a set, but this was never actually used. As
the only other feature (storing a `ProgramPoint` in a data member) was
very trivial, I replaced this class with a plain `NodeBuilder` in the
only location that used it.

GitOrigin-RevId: 41de4b1c6fb86cc09079c9e345f4cc5000c373b4