perf trace/scripting: Remove extraneous header read

perf_header__read() is already done in perf_session__open(), so
remove it from the script gen case.

Signed-off-by: Tom Zanussi <tzanussi@gmail.com>
Cc: fweisbec@gmail.com
Cc: rostedt@goodmis.org
LKML-Reference: <1267599873-8193-2-git-send-email-tzanussi@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>