As of GNU libc 2.29, <sys/mount.h> declares all the constants and
functions we need, so there's no use in including <linux/fs.h> anymore.
This silences annoying warnings like this one:

  In file included from nix/libstore/local-store.cc:32:0:
  /gnu/store/…-linux-libre-headers-4.19.56/include/linux/fs.h:108:0: warning: "MS_RDONLY" redefined
   #define MS_RDONLY  1 /* Mount read-only */

  In file included from nix/libstore/local-store.cc:28:0:
  /gnu/store/…-glibc-2.29/include/sys/mount.h:36:0: note: this is the location of the previous definition
   #define MS_RDONLY MS_RDONLY

* config-daemon.ac: Remove check for <linux/fs.h>.
* nix/libstore/build.cc: Remove conditional inclusion of <linux/fs.h>.
* nix/libstore/local-store.cc: Remove "#if HAVE_LINUX_FS_H" and
inclusion of <linux/fs.h>.