libfuzzer: Remove HEAD clang step from getting started documentation.
libfuzzer: Remove HEAD clang step from getting started documentation.

R=aizatsky@chromium.org,mmoroz@chromium.org
BUG=539572

Review URL: https://codereview.chromium.org/1879633002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#386645}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk