Remove duplicated enum, use the authoritative one.

Running a diff against lldb-x86-register-enums.h and the file
modified in this patch, the two enums were completely identical.

Deleting one of them to reduce code noise.

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@222478 91177308-0d34-0410-b5e6-96231b3b80d8