We used to have an optimized version for ToString on number nodes
which was allocating an object on the heap, therefore
preventing this code from being executed on the compiler thread.
Octane benchmark results show insignificant increase in performance
(< 0.5%) without this optimization - see
https://docs.google.com/spreadsheets/d/1MC5NrMoMSsqxZqw0ojoZvomBb7q2EOt1S0sFoJ8ld2c/edit#gid=1732639373
which leads to the conclusion we can safely remove the optimization for now.

Bug: v8:7790
Change-Id: Ia1d53608f8d10ba20e0ff57cccb34583655382c6
Reviewed-on: https://chromium-review.googlesource.com/1139063
Commit-Queue: Maya Lekova <mslekova@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54495}