Remove note about lookbehind not being supported in Safari

[Aloso]

Describe the bug

Lookbehind is supported in Safari since March. It should be safe to remove the warning soon.

To Reproduce

<< 'test'

warning(P0400): Lookbehind is not supported in all browsers, e.g. Safari

help: Avoid lookbehind if the regex should work in different browsers

Expected behavior

No warning