arch: Eliminate the vecregs.hh switching header file.

Delete the headers that are in each arch, and remove it from the list of
switching header files in arch/SConscript.

Change-Id: Ic0089f83e83f76ad6308ecfb5007b5ef11e1d949
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/50256
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/arch/SConscript

@@ -61,7 +61,6 @@ env.TagImplies(env.subst('${CONF["TARGET_ISA"]} isa'), 'gem5 lib')
 env.SwitchingHeaders(
     Split('''
         isa.hh
-        vecregs.hh
         '''),
     env.subst('${CONF["TARGET_ISA"]}'))