K6_BROWSER_ENABLED flag is redundant with browser type scenario option

[ka3de]

Currently, as of commit a8ebe8c, the K6_BROWSER_ENABLED environment variable is redundant with the "browser type" option defined in the scenarios specification, as both of these are required in order to run a test that uses the browser module, being the latter one more restrictive as it must be set per scenario.
This is due to the changes applied in #3036 to support browser options inside scenarios definition in k6 core, and the modifications done in grafana/xk6-browser#944 in order to automatically initialize browsers based on this option.

From the k6 browser team we think that the usage of this flag is no longer required, and removing it will provide a better UX.