tests: Delete the now unused unittest/unittest.[cc|hh].

These files were originally used to provide a more gtest like mechanism
for the UnitTest executables, many of which didn't actually test
anything. With the definitions in those files, the tests could check
whether their expectations were met, and either pass or fail without a
human having to inspect the output and knowing what output to expect.

Change-Id: Ie0601391b994859eb544b37201333838fa3ba02a
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40618
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>

src/unittest/SConscript

@@ -28,8 +28,6 @@
 
 Import('*')
 
-Source('unittest.cc')
-
 UnitTest('nmtest', 'nmtest.cc')
 
 stattest_py = PySource('m5', 'stattestmain.py', tags='stattest')