gdb: Remove unused extra_lines variable

Clang generates a warning if there is a variable that is set but not used
otherwise ("-Wunused-but-set-variable").  On the default configuration, it
causes a build failure (unless "--disable-werror" is specified).

The only extra_lines use in arrange_linetable function is removed on the
commit 558802e
("gdb: change subfile::line_vector to an std::vector").  So, this variable
should be removed to prevent a build failure.

gdb/xcoffread.c

@@ -419,8 +419,6 @@ add_stab_to_list (char *stabname, struct pending_stabs **stabvector)
 static void
 arrange_linetable (std::vector<linetable_entry> &old_linetable)
 {
-  int extra_lines = 0;
-
   std::vector<linetable_entry> fentries;
 
   for (int ii = 0; ii < old_linetable.size (); ++ii)
@@ -436,12 +434,6 @@ arrange_linetable (std::vector<linetable_entry> &old_linetable)
 	  e.line = ii;
 	  e.is_stmt = 1;
 	  e.pc = old_linetable[ii].pc;
-
-	  /* If the function was compiled with XLC, we may have to add an
-	     extra line entry later.  Reserve space for that.  */
-	  if (ii + 1 < old_linetable.size ()
-	      && old_linetable[ii].pc != old_linetable[ii + 1].pc)
-	    extra_lines++;
 	}
     }