Remove chain.set_coinbase & .set_extra_data: YAGNI

It was untested, and had a bug of not reusing the values on the next
header. Move to another PR, and add tests. But probably not until
someone actually starts using it.

Author: carver

eth/abc.py

@@ -4073,20 +4073,6 @@ def set_header_timestamp(self, timestamp: int) -> None:
         """
         ...
 
-    @abstractmethod
-    def set_coinbase(self, coinbase: Address) -> None:
-        """
-        Set the coinbase for all new headers.
-        """
-        ...
-
-    @abstractmethod
-    def set_extra_data(self, extra_data: bytes) -> None:
-        """
-        Set the extra_data (sometimes called graffiti) for all new headers.
-        """
-        ...
-
     @abstractmethod
     def mine_all(
             self,

eth/chains/base.py

@@ -57,9 +57,7 @@
 )
 from eth.constants import (
     EMPTY_UNCLE_HASH,
-    GENESIS_EXTRA_DATA,
     MAX_UNCLE_DEPTH,
-    ZERO_ADDRESS,
 )
 
 from eth.db.chain import (
@@ -628,8 +626,6 @@ class MiningChain(Chain, MiningChainAPI):
 
     def __init__(self, base_db: AtomicDatabaseAPI, header: BlockHeaderAPI = None) -> None:
         super().__init__(base_db)
-        self._header_extra_data = GENESIS_EXTRA_DATA
-        self._header_coinbase = ZERO_ADDRESS
         self.header = self.ensure_header(header)
 
     def apply_transaction(self,
@@ -667,14 +663,6 @@ def import_block(self,
     def set_header_timestamp(self, timestamp: int) -> None:
         self.header = self.header.copy(timestamp=timestamp)
 
-    def set_coinbase(self, coinbase: Address) -> None:
-        self.header = self.header.copy(coinbase=coinbase)
-        self._header_coinbase = coinbase
-
-    def set_extra_data(self, extra_data: bytes) -> None:
-        self.header = self.header.copy(extra_data=extra_data)
-        self._header_extra_data = extra_data
-
     def mine_all(
             self,
             transactions: Sequence[SignedTransactionAPI],