gn2bp: remove script from sources

Actions using template "action_with_pydeps" put the script inside
inputs.
In soong, a file cannot be in both tool_files and srcs, so remove it
from the latter if it is there.

This got me to successfully build a valid Android.bp file for the first
time! (All the way to the first compile error, yay!) Note that this only
works with perfetto removed via 'enable_base_tracing = false' in gn
args.
Also note that likely none of the actions are correct yet.
{{response_file_name}} tags are not handled yet and all script args that
reference a file or directory are most certainly wrong.

Test: //base:base
Change-Id: I779141121c63982f001fb82a9705439cde36dd48