os/fs.c: remove os_file_is_readonly()

os_file_is_readonly() in its current form is equivalent to
!os_file_is_writable(). This does not appear to be a bug, because Vim's
use of check_file_readonly() (which we changed to os_file_is_readonly())
is equivalent to !os_file_is_writable() in every case.

os_file_is_readonly() also fails this test:

    returns false if the file is non-read, non-write

A more useful form would define behavior under these cases:

  - path is executable (but not writable)
  - path is non-existent
  - path is directory

But there is no reason for os_file_is_readonly() to exist, so remove it.

src/nvim/os/fs.c

@@ -270,15 +270,6 @@ bool os_file_exists(const char_u *name)
   return os_stat((char *)name, &statbuf);
 }
 
-/// Check if a file is readonly.
-///
-/// @return `true` if `name` is readonly.
-bool os_file_is_readonly(const char *name)
-  FUNC_ATTR_NONNULL_ALL
-{
-  return access(name, W_OK) != 0;
-}
-
 /// Check if a file is readable.
 ///
 /// @return true if `name` is readable, otherwise false.