Previously we didn't implement TRACE_STR_COPY when we write trace events to
file, which causes us to allocate a growing independent memory chunk for dumped
runtime call stats table. Since we now have a fully functional TRACE_STR_COPY,
this memory allocation can be avoided, this patch removes it.

BUG=v8:5089

Review-Url: https://codereview.chromium.org/2342643004
Cr-Commit-Position: refs/heads/master@{#39462}