Remove some useless typedef's.

Summary:
This does not have any functionnal difference but I found myself
searching for 'struct RegisterInfo$' to find a definition and couldn't
find it. The version without typedef seems more common so let's try and
be consistent.

Reviewers: clayborg, zturner

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D14091

llvm-svn: 251351