lib: remove unnecessary id comparison

If we look at the generation of the list item that is inserted into the priority queue, we can observe that we are doing ++id before inserting or updating the value in the list, and id is a global object. Thus it is safe to say that no two element id can be equal. Also in the walk, we are iterating from root to leaf nodes, so always comparing two different elements.

Benchmark results

 timers/immediate.js type='breadth' n=5000000                                 4.09 %       ±7.09% ±9.44% ±12.30%
 timers/immediate.js type='breadth1' n=5000000                                0.42 %       ±3.96% ±5.29%  ±6.90%
 timers/immediate.js type='breadth4' n=5000000                               -1.32 %       ±2.57% ±3.42%  ±4.45%
 timers/immediate.js type='clear' n=5000000                           **     -4.10 %       ±2.54% ±3.40%  ±4.46%
 timers/immediate.js type='depth' n=5000000                                   0.23 %       ±1.59% ±2.12%  ±2.76%
 timers/immediate.js type='depth1' n=5000000                                 -0.72 %       ±1.62% ±2.15%  ±2.80%
 timers/set-immediate-breadth-args.js n=5000000                              -2.87 %       ±6.23% ±8.29% ±10.79%
 timers/set-immediate-breadth.js n=10000000                                   2.16 %       ±5.94% ±7.91% ±10.29%
 timers/set-immediate-depth-args.js n=5000000                                 0.66 %       ±2.48% ±3.32%  ±4.37%
 timers/timers-breadth-args.js n=1000000                                      1.64 %       ±3.89% ±5.18%  ±6.74%
 timers/timers-breadth.js n=5000000                                          -2.47 %       ±6.48% ±8.63% ±11.23%
 timers/timers-cancel-pooled.js n=5000000                                    -0.30 %       ±3.37% ±4.48%  ±5.83%
 timers/timers-cancel-unpooled.js direction='end' n=1000000                   2.31 %       ±4.31% ±5.74%  ±7.46%
 timers/timers-cancel-unpooled.js direction='start' n=1000000                -0.29 %       ±3.16% ±4.21%  ±5.49%
 timers/timers-depth.js n=1000                                               -0.33 %       ±1.61% ±2.14%  ±2.78%
 timers/timers-insert-pooled.js n=5000000                                     3.67 %       ±4.11% ±5.47%  ±7.13%
 timers/timers-insert-unpooled.js direction='end' n=1000000          ***      5.87 %       ±1.09% ±1.46%  ±1.91%
 timers/timers-insert-unpooled.js direction='start' n=1000000                -0.69 %       ±1.42% ±1.90%  ±2.48%
 timers/timers-timeout-nexttick.js n=50000                                    2.17 %       ±4.95% ±6.58%  ±8.56%
 timers/timers-timeout-nexttick.js n=5000000                                  0.10 %       ±5.32% ±7.08%  ±9.21%
 timers/timers-timeout-pooled.js n=10000000                                   3.32 %       ±4.41% ±5.88%  ±7.68%
 timers/timers-timeout-unpooled.js n=1000000                                  1.19 %       ±2.77% ±3.69%  ±4.82%

Be aware that when doing many comparisons the risk of a false-positive
result increases. In this case there are 22 comparisons, you can thus
expect the following amount of false-positive results:
  1.10 false positives, when considering a   5% risk acceptance (*, **, ***),
  0.22 false positives, when considering a   1% risk acceptance (**, ***),
  0.02 false positives, when considering a 0.1% risk acceptance (***)

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines