Remove UnknownPartitionStrategy error.

This became unused in #112053, when `-Zcgu-partitioning-strategy` was
removed.

Author: nnethercote

Diff for: compiler/rustc_monomorphize/messages.ftl

@@ -27,8 +27,6 @@ monomorphize_type_length_limit = reached the type-length limit while instantiati
 monomorphize_unknown_cgu_collection_mode =
     unknown codegen-item collection mode '{$mode}', falling back to 'lazy' mode
 
-monomorphize_unknown_partition_strategy = unknown partitioning strategy
-
 monomorphize_unused_generic_params = item has unused generic parameters
 
 monomorphize_written_to_path = the full type name has been written to '{$path}'

Diff for: compiler/rustc_monomorphize/src/errors.rs

@@ -75,10 +75,6 @@ pub struct LargeAssignmentsLint {
     pub limit: u64,
 }
 
-#[derive(Diagnostic)]
-#[diag(monomorphize_unknown_partition_strategy)]
-pub struct UnknownPartitionStrategy;
-
 #[derive(Diagnostic)]
 #[diag(monomorphize_symbol_already_defined)]
 pub struct SymbolAlreadyDefined {