arch: Remove the "process.hh" switching header file.

This file is not included anywhere in gem5.

Change-Id: I936ac482b9b1d527f141267d0dfb86dda3de34df
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/26235
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>

src/arch/SConscript

@@ -63,7 +63,6 @@ env.SwitchingHeaders(
         kernel_stats.hh
         locked_mem.hh
         microcode_rom.hh
-        process.hh
         pseudo_inst.hh
         registers.hh
         remote_gdb.hh