libbrillo: remove invocation of g_type_init()

g_type_init() has been deprecated (and also marked with the attribute
'deprecated') since glib 2.36 as the glib type system is now
automatically initialized. This CL removes unnecessary invocations of
g_type_init() and related comments in the code.

BUG=chromium:680616
CQ-DEPEND=CL:436384
TEST=Ran libbrillo unit tests.
TEST=Verified that CrOS DBus daemons based on libbrillo worked.

Change-Id: Iac630d3d0b75f47e3d54826bd5ac0bb2e91c79c9
Reviewed-on: https://chromium-review.googlesource.com/436584
Commit-Ready: Ben Chan <benchan@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
4 files changed