scons: Stop generating THE_ISA_STR in config/the_isa.hh.

This macro is no longer used or needed.

Jira Issue: https://gem5.atlassian.net/browse/GEM5-1056

Change-Id: Ib90f739c2d0df4c655239e55ecfe0da486ee3bf7
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/48885
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/SConscript

@@ -769,7 +769,6 @@ def makeTheISA(source, target, env):
 
 #define THE_ISA ${{define(target_isa)}}
 #define TheISA ${{namespace(target_isa)}}
-#define THE_ISA_STR "${{target_isa}}"
 
 #endif // __CONFIG_THE_ISA_HH__''')