Just a minor cleanup of ada-lang.c to remove a function that
duplicates another function implemented elsewhere.

2009-04-16  Joel Brobecker  <brobecker@adacore.com>
 
        * ada-lang.c (extract_string): Delete.
        (ada_main_name): Reimplement using target_read_string instead of
        extract_string.

This is exercised everytime we do a "start", which looks for the name
of the main routine.

Tested on x86_64-linux, no regression. Checked in.

-- 
Joel