Fix #1654

Avoid unviewable most popular forum caching by checking them inside the
function instead of relying on a global variable available only in
stats.php (while the function is used in several places).

https://github.com/mybb/mybb/issues/1654
Ah, forgot about that. Caching it for xx minutes is pointless then since every user may have other forum as value. Any suggestion of possible solution other than removing it from cache ?

@Sama34, umm, it was, just look at the PR: https://github.com/mybb/mybb/pull/1655/files#diff-dca12e71fbca629303266a3c00e84db5L663
Except the variable was not even globalised and was empty all the time.