Removed overly-specific handling of handlebars errors

refs: #10496
refs: handlebars-lang/handlebars.js#1548

- For a while, handlebars would and could generate quite a lot of obscure sytnax error messages for malformed helpers that we couldn't catch in gscan
- This was fixed in handlebars-lang/handlebars.js#1548
- We have also since added AST parsing to gscan, so if there are more obscure syntax errors, we should be able to add rules to catch them
- Therefore, this additional, overly specific code block isn't needed anymore, so let's clean it up!

core/server/web/shared/middleware/error-handler.js

@@ -105,14 +105,6 @@ _private.prepareError = (err, req, res, next) => {
             err = new errors.NotFoundError({
                 err: err
             });
-        } else if (err.stack.match(/node_modules\/handlebars\//)) {
-            // Temporary handling of theme errors from handlebars
-            // @TODO remove this when #10496 is solved properly
-            err = new errors.IncorrectUsageError({
-                err: err,
-                message: err.message,
-                statusCode: err.statusCode
-            });
         } else {
             err = new errors.GhostError({
                 err: err,