Removes the following functions:
Flush
AppendChars
WriteAsCFile (only from header since impl was already removed)

and moves local function AppendChars into anonymous namespace block.

Bug: v8:9810
Change-Id: Icc3ca8458eed4711f25514ac71aa0e6b413ed281
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1921797
Auto-Submit: Dan Elphick <delphick@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65292}