Do not require that unsafe fields lack drop glue

Instead, we adopt the position that introducing an `unsafe` field
itself carries a safety invariant: that if you assign an invariant
to that field weaker than what the field's destructor requires,
you must ensure that field is in a droppable state in your
destructor.

See:
- https://github.com/rust-lang/rfcs/pull/3458#discussion_r1971676100
- https://rust-lang.zulipchat.com/#narrow/channel/213817-t-lang/topic/unsafe.20fields.20RFC/near/502113897

compiler/rustc_hir_analysis/src/errors.rs

@@ -710,17 +710,6 @@ pub(crate) struct InvalidUnionField {
     pub note: (),
 }
 
-#[derive(Diagnostic)]
-#[diag(hir_analysis_invalid_unsafe_field, code = E0740)]
-pub(crate) struct InvalidUnsafeField {
-    #[primary_span]
-    pub field_span: Span,
-    #[subdiagnostic]
-    pub sugg: InvalidUnsafeFieldSuggestion,
-    #[note]
-    pub note: (),
-}
-
 #[derive(Diagnostic)]
 #[diag(hir_analysis_return_type_notation_on_non_rpitit)]
 pub(crate) struct ReturnTypeNotationOnNonRpitit<'tcx> {
@@ -742,18 +731,6 @@ pub(crate) struct InvalidUnionFieldSuggestion {
     pub hi: Span,
 }
 
-#[derive(Subdiagnostic)]
-#[multipart_suggestion(
-    hir_analysis_invalid_unsafe_field_sugg,
-    applicability = "machine-applicable"
-)]
-pub(crate) struct InvalidUnsafeFieldSuggestion {
-    #[suggestion_part(code = "std::mem::ManuallyDrop<")]
-    pub lo: Span,
-    #[suggestion_part(code = ">")]
-    pub hi: Span,
-}
-
 #[derive(Diagnostic)]
 #[diag(hir_analysis_return_type_notation_equality_bound)]
 pub(crate) struct ReturnTypeNotationEqualityBound {