Remove the obsolete code coverage documentation.

docs/code_coverage.md

-# Code Coverage
-
-## Categories of coverage
-
-*   executed - this line of code was hit during execution
-*   instrumented - this line of code was part of the compilation unit, but not
-    executed
-*   missing - in a source file, but not compiled.
-*   ignored - not an executable line, or a line we don't care about
-
-Coverage is calculated as `exe / (inst + miss)`. In general, lines that are in
-`miss` should be ignored, but our exclusion rules are not good enough.
-
-## Buildbots
-
-Buildbots are currently on the
-[experimental waterfall](http://build.chromium.org/buildbot/waterfall.fyi/waterfall).
-The coverage figures they calculate come from running some subset of the
-chromium testing suite.
-
-*   [Linux](http://build.chromium.org/buildbot/waterfall.fyi/builders/Linux%20Coverage%20(dbg))
-    - uses `gcov`
-*   [Windows](http://build.chromium.org/buildbot/waterfall.fyi/builders/Win%20Coverage%20%28dbg%29)
-*   [Mac](http://build.chromium.org/buildbot/waterfall.fyi/builders/Mac%20Coverage%20%28dbg%29)
-
-Also,
-
-*   [Coverage dashboard](http://build.chromium.org/buildbot/coverage/)
-*   [Example coverage summary](http://build.chromium.org/buildbot/coverage/linux-debug/49936/)
-    - the coverage is calculated at directory and file level, and the directory
-    structure is navigable via the **Subdirectories** table.
-
-## Calculating coverage locally
-
-TODO
-
-## Advanced Tips
-
-Sometimes a line of code should never be reached (e.g., `NOTREACHED()`). These
-can be marked in the source with `// COV_NF_LINE`. Note that this syntax is
-exact.