Simplify proto descriptor rules

Remove a couple of unnecessary path mangling steps and
fix ftrace.descriptor generation to be in the correct place.

Bug: 139319295
Change-Id: I7165ee70f8051ef50c511e17c3dbf47986ec1f1a

tools/run_ftrace_proto_gen

diff --git a/tools/run_ftrace_proto_gen b/tools/run_ftrace_proto_gen
index 3e9141f..c8e9ca7 100755
--- a/tools/run_ftrace_proto_gen
+++ b/tools/run_ftrace_proto_gen
@@ -16,7 +16,7 @@
   fi
 fi
 
-DESCRIPTOR='gen/protos/perfetto/trace/ftrace.descriptor'
+DESCRIPTOR='gen/protos/perfetto/trace/ftrace/ftrace.descriptor'
 "$DIR/ninja" -C "$BUILDDIR" ftrace_proto_gen $DESCRIPTOR
 
 # FIXME(fmayer): make ftrace_proto_gen independent of cwd.