alpha: Remove ALPHA tru64 support and associated tests

No one appears to be using it, and it is causing build issues
and increases the development and maintenance effort.

src/arch/alpha/AlphaSystem.py

@@ -54,9 +54,3 @@ class FreebsdAlphaSystem(AlphaSystem):
     cxx_header = "arch/alpha/freebsd/system.hh"
     system_type = 34
     system_rev = 1 << 10
-
-class Tru64AlphaSystem(AlphaSystem):
-    type = 'Tru64AlphaSystem'
-    cxx_header = "arch/alpha/tru64/system.hh"
-    system_type = 12
-    system_rev = 2<<1