Remove symbol_get_demangled_name

Remove symbol_get_demangled_name

Now that symbol_get_demangled_name is only used by general_symbol_info
methods, and because these methods already check the symbol's language
to decide what to return, symbol_get_demangled_name is no longer
needed.  This patch removes it.

gdb/ChangeLog
2020-04-24  Tom Tromey  <tom@tromey.com>

	* symtab.h (symbol_get_demangled_name): Don't declare.
	* symtab.c (symbol_get_demangled_name): Remove.
	(general_symbol_info::natural_name)
	(general_symbol_info::demangled_name): Update.