Remove 'read' call from dwz_file::read_string

dwz_file::read_string calls 'read' on the section, but this isn't
needed as the sections have all been pre-read.

This patch makes this change, and refactors dwz_file a bit to make
this more obvious -- by making it clear that only the "static
constructor" can create a dwz_file.

Approved-By: Simon Marchi <simon.marchi@efficios.com>
Tested-By: Alexandra Petlanova Hajkova <ahajkova@redhat.com>
3 files changed