remove block_fetcher and tx_fetcher (#3024)

eth/fetcher/block_fetcher.go

eth/fetcher/block_fetcher_test.go

eth/fetcher/tx_fetcher.go

eth/fetcher/tx_fetcher_test.go

...fetcher/corpus/0151ee1d0db4c74d3bcdfa4f7396a4c8538748c9-2

...fetcher/corpus/020dd7b492a6eb34ff0b7d8ee46189422c37e4a7-6

...fetcher/corpus/021d1144e359233c496e22c3250609b11b213e9f-4

...fetcher/corpus/0d28327b1fb52c1ba02a6eb96675c31633921bb2-2

...fetcher/corpus/0fcd827b57ded58e91f7ba2ac2b7ea4d25ebedca-7

...fetcher/corpus/109bc9b8fd4fef63493e104c703c79bc4a5e8d34-6

...fetcher/corpus/163785ab002746452619f31e8dfcb4549e6f8b6e-6

...fetcher/corpus/1adfa6b9ddf5766220c8ff7ede2926ca241bb947-3

...fetcher/corpus/1b9a02e9a48fea1d2fc3fb77946ada278e152079-4

...fetcher/corpus/1e14c7ea1faef92890988061b5abe96db7190f98-7

...fetcher/corpus/1e7d05f00e99cbf3ff0ef1cd7ea8dd07ad6dff23-6

...fetcher/corpus/1ec95e347fd522e6385b5091aa81aa2485be4891-4

...fetcher/corpus/1fbfa5d214060d2a0905846a589fd6f78d411451-4

...fetcher/corpus/1fd84ee194e791783a7f18f0a6deab8efe05fc04-2

...fetcher/corpus/21e76b9fca21d94d97f860c1c82f40697a83471b-8

...fetcher/corpus/220a87fed0c92474923054094eb7aff14289cf5e-4