l_mainmenu.h: remove unused l_get_dirlist function

l_mainmenu.h: remove unused l_get_dirlist function

The commit
8f9af57314f71aae1cc77e13f9996e13015d776d "Add core.get_dir_list" by @ShadowNinja
has removed the implementation of the l_get_dirlist function and all its usages
from the l_mainmenu.cpp file, but hasn't removed it from the header file.

The reason why this hasn't been detected earlier is that C++ has this interesting
feature to still make it possible to create instances of classes whose never used
private methods are declared but not defined.