Remove mapped_index_base

The base class mapped_index_base is no longer needed.  Previously it
was used by both the .gdb_index and .debug_names readers, but the
latter now uses the cooked index instead.

This patch removes mapped_index_base, merging it into
mapped_gdb_index.  Supporting code that is specific to .gdb_index is
also moved into read-gdb-index.c.  This shrinks dwarf2/read.c a bit,
which is nice.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32504
Approved-By: Andrew Burgess <aburgess@redhat.com>