gdb: delete a couple of declarations from f-lang.h

One declaration in f-lang.h is for a function that doesn't even exist,
another is for a function that is only used within f-lang.c.

One declaration is deleted, the other function I make static in
f-lang.c.

gdb/ChangeLog:

	* f-lang.c (fortran_argument_convert): Add declaration.  Add
	header comment, taken from f-lang.h.  Make static.
	* f-lang.h (f77_get_dynamic_array_length): Delete declaration.
	(fortran_argument_convert): Delete declaration.