fdo#74425 Remove deprecated static function ResMgr::GetLang()

ResMgr::GetLang() has been deprecated for a long time now. It is used
by only one function,
SubstitutePathVariables::SetPredefinedPathVariables() in the framework
module. I have therefore removed it from ResMgr and placed it as a
function in framework/source/services/substitutepathvars.cxx where it
is actually used.

Change-Id: I5f0d8f701aa45f8653020affeff6339f8fc9bc0e
Reviewed-on: https://gerrit.libreoffice.org/7791
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>