Remove some references to GNATS. Reference to gnats.html still to be purged.

From-SVN: r67280

gcc/doc/sourcebuild.texi

@@ -466,10 +466,10 @@ following are also necessary:
 
 @itemize @bullet
 @item
-At least one GNATS category for bugs in that front end and runtime
+At least one Bugzilla component for bugs in that front end and runtime
 libraries.  This category needs to be mentioned in
 @file{gcc/gccbug.in}, and in @file{gnats.html} on the GCC web site, as
-well as being added to the GNATS database.
+well as being added to the Bugzilla database.
 @item
 Normally, one or more maintainers of that front end listed in
 @file{MAINTAINERS}.