Remove is_lint field from Level::Error.

Because it's redundant w.r.t. `Diagnostic::is_lint`, which is present
for every diagnostic level.

`struct_lint_level_impl` was the only place that set the `Error` field
to `true`, and it's also the only place that calls
`Diagnostic::is_lint()` to set the `is_lint` field.

Author: nnethercote

src/diagnostics.rs

@@ -454,7 +454,7 @@ pub fn report_msg<'tcx>(
     let span = stacktrace.first().map_or(DUMMY_SP, |fi| fi.span);
     let sess = machine.tcx.sess;
     let level = match diag_level {
-        DiagLevel::Error => Level::Error { lint: false },
+        DiagLevel::Error => Level::Error,
         DiagLevel::Warning => Level::Warning(None),
         DiagLevel::Note => Level::Note,
     };