Rollup merge of rust-lang#97865 - lcnr:bb-BorrowckMode, r=Dylan-DPC

remove `BorrowckMode`

dead code after rust-lang#95565

Author: Dylan-DPC

compiler/rustc_session/src/config.rs

@@ -547,23 +547,6 @@ pub enum PrintRequest {
     LinkArgs,
 }
 
-#[derive(Copy, Clone)]
-pub enum BorrowckMode {
-    Mir,
-    Migrate,
-}
-
-impl BorrowckMode {
-    /// Returns whether we should run the MIR-based borrow check, but also fall back
-    /// on the AST borrow check if the MIR-based one errors.
-    pub fn migrate(self) -> bool {
-        match self {
-            BorrowckMode::Mir => false,
-            BorrowckMode::Migrate => true,
-        }
-    }
-}
-
 pub enum Input {
     /// Load source code from a file.
     File(PathBuf),