Remove unused variable.

Looks like we're not using this any more, so rather than add risc-v,
let's just remove this list.

Test: treehugger
Change-Id: I24bb22235c55d48c7dcbae94514ce8dcd03521c3
diff --git a/libc/kernel/tools/defaults.py b/libc/kernel/tools/defaults.py
index 99bbc3e..5fe907c 100644
--- a/libc/kernel/tools/defaults.py
+++ b/libc/kernel/tools/defaults.py
@@ -5,9 +5,6 @@
 import time, os, sys
 from utils import *
 
-# the list of supported architectures
-kernel_archs = [ 'arm', 'arm64', 'x86' ]
-
 # the list of include directories that belong to the kernel
 # tree. used when looking for sources...
 kernel_dirs = [ "linux", "asm", "asm-generic", "mtd" ]