shared/install: remove two conditionals which are always false

The name argument in UnitFileInstallInfo (i->name) should always be a unit
file name, so the conditional always takes the 'else' branch.

The only call chain that links to find_symlinks_fd() is unit_file_lookup_state
→ find_symlinks_in_scope → find_symlinks → find_symlinks_fd. But
unit_file_lookup_state calls unit_name_is_valid(name), and then name is used
to construct the UnitFileInstallInfo object in install_info_discover, which just
uses the name it was given.