Don't use SkinBuildSidebar hook for other projects feature

Using SkinBuildSidebar for this does not work with $wgEnableSidebarCache
since that is a shared cache across all pages on a wiki.

On wmf wikis, most currently have $wgEnableSidebarCache disabled
so this change shouldn't be significant performance-wise, although
it might be nice to add extra per-page caching for this part of
the sidebar.

Bug: 70234
Change-Id: Ib614808443afe993ed746c6fc57352d60015b7f9
(cherry picked from commit 3980fbe0244f8673aeaa4623d2d1317e0fd96085)
Merge "Don't use SkinBuildSidebar hook for other projects feature" into mw1.24-wmf19