no libc filename fallback on the BSDs

See there:

https://github.com/borgbackup/borg/issues/5870#issuecomment-869350973

BORG_LIBC can now be used to give the filename, thus a likely wrong fallback is not needed any more.

src/borg/xattr.py

@@ -73,8 +73,6 @@ if libc_name is None:
     # so we can only try some "usual" names for the C library:
     if sys.platform.startswith('linux'):
         libc_name = 'libc.so.6'
-    elif sys.platform.startswith(('freebsd', 'netbsd')):
-        libc_name = 'libc.so'
     elif sys.platform == 'darwin':
         libc_name = 'libc.dylib'
     else: