scons: Remove an error check from the ProtoBuf declare-er.
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 <8eef13a4f970e2e9372c8d26ad0737b760f99c02@arm.com>
Reviewed-by: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Reviewed-by: Bobby Bruce <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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