util: Delete the execfile m5 utility command.
util: Delete the execfile m5 utility command.

This command was just a thin wrapper around the readfile command, and
can be easily duplicated by piping the output of that command to
/tmp/execfile and then running it.

Change-Id: Ib77166ee9f07571d57e4d21e9615def55e5391a8
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27233
Maintainer: Gabe Black <5c8aa1266c03186393d9e7b3b4e3b14239af8201@google.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Bobby R. Bruce <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5