[sanitizer] Remove empty Symbolizer PrepareForSandboxing

Summary:
`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.

Reviewers: alekseyshl, eugenis, dvyukov, mcgrathr

Reviewed By: alekseyshl

Subscribers: kubamracek, delcypher, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D44953

llvm-svn: 329094