Delete free_named_symtabs and associated cleanup.

Delete free_named_symtabs and associated cleanup.

        * symfile.h (free_named_symtabs): Delete declaration.
        * symfile.c: Remove some commented out code (clear_symtab_users_once).
        (cashier_psymtab): Comment function out.
        Delete declaration.
        (free_named_symtabs): Delete.
        * coffread.c (coff_end_symtab): Remove call to free_named_symtabs.
        * dbxread.c (end_psymtab): Likewise.
        * dwarf2read.c (process_psymtab_comp_unit): Ditto.
        * exec.c (exec_close_1): Ditto.
        * xcoffread.c (xcoff_end_psymtab): Likewise.