fuzzers: removed fuzzbuzz configuration (#21813)

We decided to move our fuzzing efforts to oss-fuzz since fuzzbuzz is still early access.

fuzzbuzz.yaml

-# bmt  keystore  rlp  trie  whisperv6
-
-base: ubuntu:16.04
-targets:
-  - name: rlp
-    language: go
-    version: "1.13"
-    corpus: ./fuzzers/rlp/corpus
-    harness:
-      function: Fuzz
-      package: github.com/ethereum/go-ethereum/tests/fuzzers/rlp
-      checkout: github.com/ethereum/go-ethereum/
-  - name: keystore
-    language: go
-    version: "1.13"
-    corpus: ./fuzzers/keystore/corpus
-    harness:
-      function: Fuzz
-      package: github.com/ethereum/go-ethereum/tests/fuzzers/keystore
-      checkout: github.com/ethereum/go-ethereum/
-  - name: trie
-    language: go
-    version: "1.13"
-    corpus: ./fuzzers/trie/corpus
-    harness:
-      function: Fuzz
-      package: github.com/ethereum/go-ethereum/tests/fuzzers/trie
-      checkout: github.com/ethereum/go-ethereum/
-  - name: txfetcher
-    language: go
-    version: "1.13"
-    corpus: ./fuzzers/txfetcher/corpus
-    harness:
-      function: Fuzz
-      package: github.com/ethereum/go-ethereum/tests/fuzzers/txfetcher
-      checkout: github.com/ethereum/go-ethereum/
-  - name: whisperv6
-    language: go
-    version: "1.13"
-    corpus: ./fuzzers/whisperv6/corpus
-    harness:
-      function: Fuzz
-      package: github.com/ethereum/go-ethereum/tests/fuzzers/whisperv6
-      checkout: github.com/ethereum/go-ethereum/