[clang-fuzzer] Remove Dockerfile

Was going through Dockerfiles to see where we are missing FROM lines
with fully qualified names and came across this one. I think it is safe
to say it has not been used in a very long time or maintained at all
since then since it still tries to download the source tree using svn.
Given that, delete it to lower support surface slightly.

Author: boomanaiden154

clang/tools/clang-fuzzer/Dockerfile

@@ -99,9 +99,6 @@ Example:
     -DCLANG_ENABLE_PROTO_FUZZER=ON
   ninja clang-proto-fuzzer clang-proto-to-cxx
 
-This directory also contains a Dockerfile which sets up all required
-dependencies and builds the fuzzers.
-
 ============================
  Running clang-proto-fuzzer
 ============================