chore: remove per-check alerts and alert routing FFs

[VikaCep]

Removes per-check alerts and alert routing preview feature flags

Since the features have been GA for some time, we no longer need to keep the feature flags.

Removed:

• sm-alerts-per-check FF
• synthetic-monitoring-alerting-routing FF

Related deployment_tools PRs:

• dev: https://github.com/grafana/deployment_tools/pull/427641
• staging: https://github.com/grafana/deployment_tools/pull/427647
• canary: https://github.com/grafana/deployment_tools/pull/427653
• prod: https://github.com/grafana/deployment_tools/pull/427655

[github-actions]

Script size changes

Name	+/-	Main	This PR	Outcome
[362.js]	=	1,715.31 kB	1,715.31 kB	✅
[602.js]	-0.06%	971.08 kB	970.46 kB	✅
[datasource/module.js]	-0.39%	24.63 kB	24.53 kB	✅
[692.js]	-0.47%	20.47 kB	20.37 kB	✅
[663.js]	=	5.84 kB	5.84 kB	✅
[module.js]	=	4.92 kB	4.92 kB	✅

Totals

Name	+/-	Main	This PR	Outcome
[Scripts]	-0.03%	2,742.25 kB	2,741.44 kB	✅
[Non-script Assets]	=	2,666.48 kB	2,666.48 kB	✅
[All]	-0.01%	5,408.73 kB	5,407.92 kB	✅

Generated by 🚫 dangerJS against 132f68d

[ckbedwell]

LGTM!