Debugger: Streamline watch editing code.

Implement WatchModel::data() to return the right values
for Qt::EditRole, obsoleting the role matching in the delegate.
Similarly, handle Qt::EditRole in setData(), leaving only
'Change watch expression' in the delegate, which removes
and recreates a row, causing crashes when done in setData().

src/plugins/debugger/debuggerconstants.h

@@ -240,8 +240,6 @@ enum ModelRoles
     RequestToggleWatchRole,
     RequestToolTipByExpressionRole,
     RequestClearCppCodeModelSnapshotRole,
-    RequestAssignValueRole,
-    RequestAssignTypeRole,
     RequestWatchExpressionRole,
     RequestRemoveWatchExpressionRole,