gdb/dwarf: remove unnecessary `this->` in read.c

I like using `this->` when it's unclear that the method or field
accessed is within the current class, but when accessing a private
member prefixed with `m_`, it's unnecessary, as the prefix makes it
clear.  Remove some instances of it (some coming from the previous
patch, other pre-existing) to de-clutter the code a bit.

Change-Id: Ia83d0bce51d222fa3ac3d756d50170ec6ed12b94
Approved-By: Tom Tromey <tom@tromey.com>