configs: Remove check for kernel in fs.py.

configs: Remove check for kernel in fs.py.

It is *not* true that a kernel is required in FS mode. For example,
in SPARC, gem5 is set up to run actual system firmware which will load
a kernel from the disk image. Other systems can run in a bare metal
mode where they also have no kernel.

If a configuration requires a kernel, it should check for it in C++
where there context lives, not globally in fs.py.

Change-Id: Ib094c29474c248f866bd08d4f975648a2c707a19
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24284
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>