Remove the omit_gdb_pretty_printer_section attribute

Disabling loading of pretty printers in the debugger itself is more
reliable. Before this commit the .gdb_debug_scripts section couldn't be
included in dylibs or rlibs as otherwise there is no way to disable the
section anymore without recompiling the entire standard library.

tests/debuginfo/generic-function.rs

@@ -1,4 +1,5 @@
 //@ compile-flags:-g
+//@ disable-gdb-pretty-printers
 
 // === GDB TESTS ===================================================================================
 
@@ -44,9 +45,6 @@
 // lldb-check:[...] { a = 6 b = 7.5 }
 // lldb-command:continue
 
-#![feature(omit_gdb_pretty_printer_section)]
-#![omit_gdb_pretty_printer_section]
-
 #[derive(Clone)]
 struct Struct {
     a: isize,