Remove la_get_string member

Remove la_get_string member

The la_get_string member of struct language_defn was intended to
provide a way to fetch string data from a "string" object in a
language-dependent way.  However, it turned out that this was never
needed, and was only ever implemented for C.  This patch removes the
language hook entirely.

gdb/ChangeLog
2019-11-05  Tom Tromey  <tom@tromey.com>

	* rust-lang.c (rust_language_defn): Update.
	* python/py-value.c (valpy_string): Call c_get_string.
	* p-lang.c (pascal_language_defn): Update.
	* opencl-lang.c (opencl_language_defn): Update.
	* objc-lang.c (objc_language_defn): Update.
	* m2-lang.c (m2_language_defn): Update.
	* language.c (unknown_language_defn, auto_language_defn): Update.
	(default_get_string): Remove.
	* guile/scm-value.c (gdbscm_value_to_string): Use c_get_string.
	* go-lang.c (go_language_defn): Update.
	* f-lang.c (f_language_defn): Update.
	* d-lang.c (d_language_defn): Update.
	* c-lang.c (c_language_defn, cplus_language_defn)
	(asm_language_defn, minimal_language_defn): Update.
	* ada-lang.c (ada_language_defn): Update.
	* language.h (struct language_defn) <la_get_string>: Remove.
	(LA_GET_STRING): Remove.
	(default_get_string): Don't declare.

Change-Id: Ia97763dfe34dc8ecb46587f7a651f8af9be8fdbd