gdb: unify two dis_asm_read_memory functions in disasm.c

After the recent restructuring of the disassembler code, GDB has ended
up with two identical class static functions, both called
dis_asm_read_memory, with identical implementations.

My first thought was to move these out of their respective classes,
and just make them global functions, then I'd only need a single
copy.

And maybe that's the right way to go.  But I disliked that by doing
that I loose the encapsulation of the method with the corresponding
disassembler class.

So, instead, I placed the static method into its own class, and had
both the gdb_non_printing_memory_disassembler and gdb_disassembler
classes inherit from this new class as an additional base-class.

In terms of code generated, I don't think there's any significant
difference with this approach, but I think this better reflects how
the function is closely tied to the disassembler.

There should be no user visible changes after this commit.

gdb/disasm.c

@@ -132,9 +132,9 @@ line_has_code_p (htab_t table, struct symtab *symtab, int line)
 /* Wrapper of target_read_code.  */
 
 int
-gdb_disassembler::dis_asm_read_memory (bfd_vma memaddr, gdb_byte *myaddr,
-				       unsigned int len,
-				       struct disassemble_info *info)
+gdb_disassembler_memory_reader::dis_asm_read_memory
+  (bfd_vma memaddr, gdb_byte *myaddr, unsigned int len,
+   struct disassemble_info *info)
 {
   return target_read_code (memaddr, myaddr, len);
 }
@@ -1021,16 +1021,6 @@ gdb_non_printing_disassembler::null_fprintf_styled_func
   return 0;
 }
 
-/* See disasm.h.  */
-
-int
-gdb_non_printing_memory_disassembler::dis_asm_read_memory
-  (bfd_vma memaddr, bfd_byte *myaddr, unsigned int length,
-   struct disassemble_info *dinfo)
-{
-  return target_read_code (memaddr, myaddr, length);
-}
-
 /* A non-printing disassemble_info management class.  The disassemble_info
    setup by this class will not print anything to the output stream (there
    is no output stream), and the instruction to be disassembled will be