arch: Delete the decoder.hh switching header file.

Change-Id: I9340aa9fa87bcb03c27cb7107b6697ad96060dbe
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/52082
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
diff --git a/src/arch/SConscript b/src/arch/SConscript
index d9c19df..3034dac 100644
--- a/src/arch/SConscript
+++ b/src/arch/SConscript
@@ -59,7 +59,6 @@
 
 env.SwitchingHeaders(
     Split('''
-        decoder.hh
         isa.hh
         vecregs.hh
         '''),