assert: remove unreachable code

There is only one entry in `kReadableOperator` that ends in `Unequal`.
So the value assigned on line 392 can only be truthy if `operator` is
`notDeepEqual`. Therefore, the ternary condition on line 394 is always
true. Remove the ternary. Coverage reports confirm that the removed code
is unused.

PR-URL: #27840
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>
Reviewed-By: Ouyang Yadong <oyydoibh@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

lib/internal/assert/assertion_error.js

@@ -391,10 +391,9 @@ class AssertionError extends Error {
           if (operator === 'deepEqual') {
             res = `${knownOperator}\n\n${res}\n\nshould loosely deep-equal\n\n`;
           } else {
-            const newOperator = kReadableOperator[`${operator}Unequal`];
-            if (newOperator) {
-              const eq = operator === 'notDeepEqual' ? 'deep-equal' : 'equal';
-              res = `${newOperator}\n\n${res}\n\nshould not loosely ${eq}\n\n`;
+            const newOp = kReadableOperator[`${operator}Unequal`];
+            if (newOp) {
+              res = `${newOp}\n\n${res}\n\nshould not loosely deep-equal\n\n`;
             } else {
               other = ` ${operator} ${other}`;
             }