Remove pragmas related to -Wunreachable-code

-Wunreachable-code has been a no-op since GCC 4.5; GRUB hasn't been
compiled with it since 2012; and GCC 6 produces "error:
'-Wunreachable-code' is not an option that controls warnings" for these.

Fixes Debian bug #812047.

grub-core/lib/pbkdf2.c

@@ -31,7 +31,6 @@ GRUB_MOD_LICENSE ("GPLv2+");
    desired derived output length DKLEN.  Output buffer is DK which
    must have room for at least DKLEN octets.  The output buffer will
    be filled with the derived data.  */
-#pragma GCC diagnostic ignored "-Wunreachable-code"
 
 gcry_err_code_t
 grub_crypto_pbkdf2 (const struct gcry_md_spec *md,

grub-core/script/parser.y

@@ -30,7 +30,6 @@
 
 #include "grub_script.tab.h"
 
-#pragma GCC diagnostic ignored "-Wunreachable-code"
 #pragma GCC diagnostic ignored "-Wmissing-declarations"
 
 %}