file,directory: Drop obsolete is_local() methods

They are plain wrappers for g_file_is_native(), and, after the previous
commits, there are only two usages, which are found in:

* window-slot.c, introduced in b3a4eabf
* files-view.c, introduced in f98d2edb

In both cases, we can just call g_file_is_native() directly. This way,
we can drop the chronically-misinterpretted is_local() methods, whose
various interpretations are now covered by specific checking methods:

* g_file_is_native()
* nautilus_file_has_local_path()
* !nautilus_file_is_remote()
2 jobs for wip/antoniof/local_vs_native_fixup in 8 minutes and 19 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Test
passed #813195
flatpak
flatpak devel

00:08:19

passed #813196
style check

00:01:31