scons: Remove an error check from the ProtoBuf declare-er.

ProtoBuf files can (and should) be declared to SCons unconditionally,
but then also use a "protobuf" tag. This lets SCons still include them
conditionally when it gathers sources to build certain binaries without
having additional python logic in SConscripts, or manually interpreting
config state.

Change-Id: I9426d647bc2d6c9d7eee7dadba77f75ed83bcaad
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/58349
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>

src/SConscript

@@ -259,11 +259,6 @@ cxx_file.add_emitter('.proto', protoc_emitter)
 
 def ProtoBuf(source, tags=None, add_tags=None):
     '''Add a Protocol Buffer to build'''
-
-    if not env['HAVE_PROTOC'] or not env['CONF']['HAVE_PROTOBUF']:
-        error('Got protobuf to build, but lacks support!')
-
-    '''Specify the source file, and any tags'''
     Source(source, tags, add_tags, append={'CXXFLAGS': '-Wno-array-bounds'})
 
 env['PROTOC_GRPC'] = distutils.spawn.find_executable('grpc_cpp_plugin')