Remove environment_metrics from helpers to avoid extra redirection

What

Avoids extra redirection on user side and accordingly removes environment_metrics_path helper method itself, which was originally introduced in gitlab-foss!8935 (merged) (for %9.0) but deprecated in gitlab-foss!12566 (merged) (%9.4).

before	after
environment_metrics_path(environment)	project_metrics_dashboard_path(project, environment: environment) (/-/metrics?environment=1)
metrics_project_environment_path(project, environment, *args) (/-/environments/1/metrics)	(not in use due to !38364 (merged))
/environments/1/metrics (hard-coded in JS)	/-/metrics?environment=1

cf. !38364 (merged)

A link changed in this MR

(This MR does not make visual changes)

Signed-off-by: Takuya Noguchi takninnovationresearch@gmail.com