lldb should not define an enum for AMDGPU targets

#206369 added a new manually defined enum listing every AMDGPU target. This is an unwelcome development that increases the effort of introducing a new target, which is a frequent occurrence. This enum also included r600 targets, which definitely aren't tested and will never work. Clang used to do something similar, but it had the excuse of an enum shared with other targets, but this was fixed in #213362.

All target information should exclusively come from TargetParser, and each llvm client shouldn't maintain its own list of targets and target information. TableGen generates the GPUKind enum, and we additionally have the Triple SubArchType entries.