[explorer, moves, position] Remove unused MoveBuilder

This thing isn't used by any of the "modern" code I've written. It was difficult
to write, but kinda neat architecturally. It made a lot of invalid configurations
of moves into build-time errors. Anyway…

Remove several of the tests that relied on it, but that hadn't been updated to
use the newer code either.

position/src/testing.rs

@@ -1,6 +1,6 @@
 // Eryn Wells <eryn@erynwells.me>
 
-use chessfriend_moves::{BuildMoveError, MakeMoveError};
+use chessfriend_moves::MakeMoveError;
 
 #[macro_export]
 macro_rules! assert_eq_bitboards {
@@ -19,17 +19,10 @@ pub type TestResult = Result<(), TestError>;
 
 #[derive(Debug, Eq, PartialEq)]
 pub enum TestError {
-    BuildMove(BuildMoveError),
     MakeMove(MakeMoveError),
     NoLegalMoves,
 }
 
-impl From<BuildMoveError> for TestError {
-    fn from(value: BuildMoveError) -> Self {
-        TestError::BuildMove(value)
-    }
-}
-
 impl From<MakeMoveError> for TestError {
     fn from(value: MakeMoveError) -> Self {
         TestError::MakeMove(value)