gdb: Remove LANG_MAGIC

gdb: Remove LANG_MAGIC

The language_defn structure has an la_magic field, this used to be
used as a basic check that the language_defn structure had the
expected layout - at least the end of the structure was where we
expected it to be.

This feature only really makes sense if we imagine GDB dynamically
loading language support from dynamic libraries, where a version
mismatch might cause problems.

However, in current GDB language support is statically built into GDB,
and since this commit:

    commit 47e77640be31fc1a4eb3718f594ed5fd0faff065
    Date:   Thu Jul 20 18:28:01 2017 +0100

        Make language_def O(1)

the existing (if pointless) check of the la_magic field was removed.

There now appears to be no use of the la_magic field, and I propose
that we delete it.

There should be no user visible changes after this commit.

gdb/ChangeLog:

	* ada-lang.c (ada_language_defn): Remove use of LANG_MAGIC.
	* c-lang.c (c_language_defn): Likewise.
	(cplus_language_defn): Likewise.
	(asm_language_defn): Likewise.
	(minimal_language_defn): Likewise.
	* d-lang.c (d_language_defn): Likewise.
	* f-lang.c (f_language_defn): Likewise.
	* go-lang.c (go_language_defn): Likewise.
	* language.c (unknown_language_defn): Likewise.
	(auto_language_defn): Likewise.
	* language.h (struct language_defn): Remove la_magic field.
	(LANG_MAGIC): Delete.
	* m2-lang.c (m2_language_defn): Remove use of LANG_MAGIC.
	* objc-lang.c (objc_language_defn): Likewise.
	* opencl-lang.c (opencl_language_defn): Likewise.
	* p-lang.c (pascal_language_defn): Likewise.
	* rust-lang.c (rust_language_defn): Likewise.