llgs: remove all Initialize() calls except for Debugger::Initialize().

See thread here:
http://lists.cs.uiuc.edu/pipermail/lldb-commits/Week-of-Mon-20140825/012580.html

The original change I made there was due to a segfault.  That was
caused by me directly calling PluginManager::Initialize (), with
PluginManager::Initialize() depending on HostInfo::Initialize () to
have been called already (which it wasn't).

The call to PluginManager::Initialize () was erroneous (at least at the
current time) since that method is already called by Debugger::Initialize()'s
implementation.

We will want to revisit initializing a smaller core of the debugger
suitable for lldb-gdbserver and lldb-platform.