arch: Remove the mt.hh switching header.

This header was only useful in MIPS and is only used internally within
MIPS. It doesn't need to be a switching header file.

Change-Id: Id7005f73b95e122f9ab83b3b657cae3391682f26
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/18468
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/arch/SConscript

@@ -67,7 +67,6 @@ env.SwitchingHeaders(
         locked_mem.hh
         microcode_rom.hh
         mmapped_ipr.hh
-        mt.hh
         process.hh
         pseudo_inst.hh
         registers.hh