Remove MetricsReportingEnabled policy

[Learpcs]

It is a chromium policy, but in brave it's no-op.

From here https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove):

Disable Reporting API. Note that the JS API for Reporting API (ReportingObserver) is enabled but is a no-op in Brave, see below.

On brave://policy it yields the following error:

Error This policy is blocked, its value will be ignored.

[JordanVegas]

fixed this here https://github.com/JordanVegas/SlimBrave-2