simplify calculate_committee_fraction (sigp#4213)

## Issue Addressed

[sigp#4211](sigp#4211)

## Proposed Changes

This PR conforms the helper function `calculate_committee_fraction` to the [v1.3.0 spec](https://github.com/ethereum/consensus-specs/blob/v1.3.0/specs/phase0/fork-choice.md#get_weight)

## Additional Info

the old definition of `calculate_committee_fraction` is almost identical, but the new definition is simpler.

consensus/proto_array/src/fork_choice_test_definition/execution_status.rs

@@ -987,11 +987,11 @@ pub fn get_execution_status_test_definition_03() -> ForkChoiceTestDefinition {
 
     ops.push(Operation::AssertWeight {
         block_root: get_root(0),
-        weight: 33_000,
+        weight: 33_250,
     });
     ops.push(Operation::AssertWeight {
         block_root: get_root(1),
-        weight: 33_000,
+        weight: 33_250,
     });
     ops.push(Operation::AssertWeight {
         block_root: get_root(2),
@@ -1000,7 +1000,7 @@ pub fn get_execution_status_test_definition_03() -> ForkChoiceTestDefinition {
     ops.push(Operation::AssertWeight {
         block_root: get_root(3),
         // This is a "magic number" generated from `calculate_committee_fraction`.
-        weight: 31_000,
+        weight: 31_250,
     });
 
     // Invalidate the payload of 3.

consensus/proto_array/src/proto_array.rs

@@ -1055,13 +1055,9 @@ pub fn calculate_committee_fraction<E: EthSpec>(
     justified_balances: &JustifiedBalances,
     proposer_score_boost: u64,
 ) -> Option<u64> {
-    let average_balance = justified_balances
+    let committee_weight = justified_balances
         .total_effective_balance
-        .checked_div(justified_balances.num_active_validators)?;
-    let committee_size = justified_balances
-        .num_active_validators
         .checked_div(E::slots_per_epoch())?;
-    let committee_weight = committee_size.checked_mul(average_balance)?;
     committee_weight
         .checked_mul(proposer_score_boost)?
         .checked_div(100)