Symbolizer::PrepareForSandboxing is empty for all platforms and apparently
has been for a while (D10213). Remove it, and shuffle things around so that the
platform specific code is now in PlatformPrepareForSandboxing.
This allows to have one less symbolizer dependency in a common file, which
helps for the upcoming split.
Also remove SymbolizerPrepareForSandboxing in tsan_go which appears to not
be used anywhere.