fs: delete unused Makefile

fs/Makefile is unused. The top-level Makefile sets LIBS-y += fs/xxx and
hence causes make to directly descend two directory levels into each
individual filesystem, and it never descends into fs/ itself.

So, delete this useless file.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Acked-by: Simon Glass <sjg@chromium.org>