*: remove generated protobufs from version control

The `go get` ship sailed long ago, when we moved to using our non-Go
dependencies' native build systems. PR cockroachdb#18349 pushed us even further in
that direction by automatically rebuilding the admin UI as necessary. We
might as well remove the generated protobuf code too; the Make rules to
generate them on-demand are already in place.

.gitattributes

@@ -1,5 +1,4 @@
 *.ir.* -diff
-*.pb.* -diff
 pkg/sql/ir/parser/irgen/irgen.go -diff
 pkg/sql/parser/help_messages.go -diff
 pkg/sql/parser/sql.go -diff

.gitignore

@@ -24,4 +24,7 @@ customenv.mk
 
 # Generated files containing include paths.
 zcgo_flags*.go
+*.pb.go
+
+# Non-distributable railroad diagram generator.
 build/Railroad.jar