Remove feature flag for APM Alert Details page (#150475)

## Summary

This PR removes the
`xpack.observability.unsafe.alertDetails.apm.enabled` feature flag to
enable the Alert Detail page for the APM Latency Threshold Rule. I've
also removed the`alertDetailsAppSection` options along with the
`alertDetailsUrl` action context variable for the remaining APM rules
since the APM Alert Detail page should only be available for the APM
Latency Threshold rule.

I also changed`isAlertDetailsEnabledPerApp()` method to allow the
`apm.transaction_duration` rule type to work but not `apm.error_rate`,
`apm.transaction_error_rate`, or `apm.anomaly` rule types. This change
was necessary because the granularity of the feature flag was set to the
solution level and didn't allow to set the flag per rule type.

### Testing

To test this PR you will need to have some APM data in your cluster.
1. Create an APM Latency Threshold Rule with a low threshold so it will
trigger
2. Create an APM Error Count Rule with a low threshold so it will
trigger
3. Click on the "Action Menu" (the ellipses) in the alert (in the Alert
Table) for the latency threshold alert
4. Click on the "View alert details" menu item for the latency threshold
alert
5. Verify it takes you to the new "alert details" page for APM
6. Repeat steps 3 & 4 for the error count alert BUT it should open the
"alert flyout" instead.

---------

Co-authored-by: Faisal Kanout <faisal.kanout@elastic.co>
Co-authored-by: kibanamachine <42973632+kibanamachine@users.noreply.github.com>

x-pack/plugins/observability/public/plugin.ts

@@ -63,9 +63,6 @@ import { registerObservabilityRuleTypes } from './rules/register_observability_r
 export interface ConfigSchema {
   unsafe: {
     alertDetails: {
-      apm: {
-        enabled: boolean;
-      };
       metrics: {
         enabled: boolean;
       };